I read he’s told 3 associates that his confidence level is close to 100% on this. Given who he is, and his background, if he were just putting together a viable suspect - he’d know it’s an attempt at convincing people at 51%, not 100. So if it’s true his colleagues really heard that from him - I suspect he’s talked to Garrett or somebody that could give some verification that the scenario fits Garrett and has more info that he’s shared.
Either Garrett or one of the other people that was there.
I can’t see Whelan going public with Garrett’s name unless he had spoken to him. That’s a slander case waiting to happen if he hasn’t cleared it.