In this case the player and the ball were in the same place, and this isn't U15 select play. Czech wasn't at a disadvantage from the lunge.
I've called this in cases where the shot came in from a steeper angle, and the offside player was just a few feet in front of the keeper. And when the shot was taken, the keeper's eyes were on the player in front of them - understandably so, since they expected the pass to go there. That's offside. Catching that is difficult but I have spotted it a couple times and pulled the goal back.