To understand the connection (or lack thereof), it's necessary to understand a bit about the nature of both your typical S&C program and the combine itself.
I have previously spoken critically of Longo's program. I do believe it to be average. But it isn't bad. He does some things I love, some things I wouldn't personally do but which are at least based on an acceptable premise, and some things that I think stink. But, for now, let's just make the broad assumption that the program as a whole is designed to develop the qualities consistent with improved performance on a football field, and that the the program succeeds in that task to an acceptable degree.
We can then comfortably say that the program is designed to improve force production (strength) and repeatable force production over a predictable rep/set/series time frame (conditioning), and that the need to develop and express those qualities will vary by position. Specifically, all football players must be able to express a lot of force, reach near-maximal force production as quickly as possible, and be able to produce that force for approximately 7 seconds per rep (a single play), for a set of 4-12 reps (a single possession), and for 10-12 total series (total number of possessions in a game).
Any training that does not promote the above is simply misguided.
Now let's look at the demands of the combine:
Bench Press -
For the typical lineman who can bench 350-400 lbs, the 225 lb bench press test represents 55-65% of maximal force production. In other words, it is very light. Thus, you see linemen testing out at 25-35 reps, and it takes approximately 20-30 seconds to complete the task. That is 3-4x as long as they are asked to produce force during a single rep in a game. The metabolic demands are entirely different. Now, if the S&C program has done its job, then the player will benefit simply by being stronger and thus utilizing a smaller percentage of max for the test. But it is still a useless expression of strength at it applies to the game of football, and thus should not be worked directly as a part of an effective S&C program. Assuming adequate strength, a player will make remarkable improvement on this test by ceasing to train for maximal force production or repeatable force production as it pertains to football, and instead using his existing strength and then developing the specific ability to express that strength in the lactic acid zone. This is counter-productive for football players, as they should never be working above lactic threshold. In other words, a lineman will get better at this test by halting (and possibly reversing) the process of developing as a football player.
For the RB/WR/DB, the 225 test is going to represent a much greater percentage of max force production. So, ironically, for a position that doesn't rely on maximal upper body force production at all, this is far more of a maximal upper body force production test than it is for the linemen. This is a useless expression of strength as it applies to the game of football. These positions are simply wasting their time and limited adaptive resources if they training for maximal upper body strength production. Again, a RB/WR/DB will improve at this test by haulting or reversing development as a football player.
40-yard dash -
Linmen rarely take more than a single step before their force production is met by opposing force production. They rarely move more than 5-10 yards at a time. When they do, it almost never linear, and their bodies are never upright. Thus, nothing done in the S&C program should be focused on the display of force production in linear, upright fashion, over the course of 40 yards. (I will say that there is plenty of reason to have linemen training to express force rapidly with only their body weight in the form of various jumping and hopping drills. So there is some applicability there to the 40 yard dash.) It is also important to note that this test is incredibly technical. I have seen players take over .2 off their times simply by changing their set-up at the starting line. They will shave additional time by learning to run relaxed, swing from the shoulder with full range of motion, drive the knee up to parallel, etc. None of these things take place on football field for linemen. Thus, they should not be a core component of the S&C program. (Another side note: I do believe it is valuable for all athletes, especially young athletes, to develop a base level of proficiency with sprint mechanics, as it teaches them to move efficiently. But this test requires far more than basic proficiency.) Now, it does tell you something if a lineman excels at this test. It tells you he is a phenomenal natural athlete, and that certainly correlates to being a good football player. But the development of that level of raw athleticism is not within the scope of the S&C program.
For DB's, this test does closely mimic the demands placed on them during the football game, and thus can be more closely connected to the efficacy of the S&C program. DB's need to be straight-line fast. This test assesses straight-line speed. But, as with the linemen, the DB's are also subject to the highly technical aspects of the test. So, the difference between a 4.8 and a 4.65 could come down to mastering the technical aspects of the test. As with the lineman 225 bench test, this test for DB's will absolutely reflect on the nature of the S&C program. But the difference between sub-par and impressive can come down to skills which are learned outside the S&C program.
This is primarily a test of raw athleticism and highly-specific technical proficiency.
Short Shuttle and 3-Cone Drills -
These are both agility tests. Like the 40, these assess raw athleticism and highly-technical proficiency. The nature of the athleticism displayed in these drills by all positions is certainly something that can, and should, be developed in a good S&C program. These drills assess the athlete's ability to produce force quickly, absorb force in good mechanical positions, and use elastic return of that force to change direction. All those qualities should be areas of emphasis in the S&C program. Unfortunately, they rarely are. But the role of technique in these drills is even greater than in the 40 yard dash. The cross-over step to start the short shuttle, the relation of the plant foot to the center of mass, the left-handed start in the 3-cone drill which ensures the optimal number of steps such that the proper pivot foot lands adjacent to the first cone; these are all techniques that can shave significant time off these drills. But technical instruction in these drills has no place in the S&C program.
Vertical Jump -
Probably the strongest correlation between success in a combine drill and adequate development in the S&C program. But, again, success in this drill is highly dependent on raw athleticism. There is also a not-insignificant amount of technique involved. But overall, for all positions, the S&C program should provide a good base (elasticity, rate of force development, flexibility in the hips) for success in this drill.
Obviously, a well-trained athlete is going to perform better during the combine than one who didn't squat, sprint, jump, bench, etc. But you have to take the combine results with a grain of salt, especially those which are actually contra-indicated for improved football performance. Of course, this doesn't even take into account the unusual combine factors such as stress, reduced sleep, "cold" performance of drills after long waits in line, etc. I'm critical of Longo, but I would never blame him for Te'o's mediocre 40 or Motta's overall poor day.
I would say most of it is simply these guys being better football players than they are raw athletes. I would also place a lot of blame on their combine prep coaches. That field is a damn racket. But that's a different conversation.