He had been questioned by the police prior to the talk with Kraft. More than likely they asked him where he had been the night before suggesting Hernandez would say he was at the club that night. Not indicating he knew when it happened because he was a part of it but because he was told.
Either way he was there so it's really a moot point but the fact the jury lacked any sort of deductive reasoning to come to that possibility is beyond me. Especially to speculate that because he knew the time he must have been the shooter when two other people were in the car.
And yes, one juror said he believed he was the killer based on that.
Let's get this part clear about the Kraft testimony:
Kraft asked him if he was involved, and Hernandez said that he hoped the time of death would come out because he was in the club at that time. Not that he was in the club that night, but that he knew when the killing occurred and had an alibi for that time.