I'm just taking it at face value. I think the congrats are sincere.
Opposing fan posts are - by their very nature - passive aggressive. He's posting on a Bills board, but of course he's going to have allegiance to his own team, and build them up in a way that rubs this fanbase the wrong way (mainly because of the team he roots for).
He's generally very cool, though. I wouldn't say that about all of the Pats fans who have spent time here. There was one who clearly came just to troll, but I haven't seen him in quite awhile.