They will now have to investigate what he said if they were seriously considering Kizer. Johnny M and too many others have been a bust based on character flaws for any GM to just dismiss what Kelly said.
Kizer is not Sanchez. Sanchez was considered polished and didn't have a down year entering the draft to fend off. Kizer has a down year and now the coach saying he may have hidden character flaws to contend with. GMs were already worried Kizer was a risk. That worry now just went up a notch and for some GMs the fear of the unknown easily will be enough to take a pass on him and take the next guy in line or hope he falls a round and they get him at half price and half risk.
It is very hard to prove a negative. Kizer can't easily prove Kelly is wrong sans getting a long line of people to go on the record with the opposite story as Kelly. That's not a given he can make that happen. A lot of people he needs to talk on his behalf are still at ND and won't go public because Kelly can still do them harm.