It made sense to Jean to assume that he reacted to April's birth by doing something hard to forgive because the stupid bastard neglected to share the fact that Fiona happened to be a relative who looked like Broderick Crawford in a fright wig. Canadian Content Cliff Clavin will gladly tell you everything that's been on his narrow, empty mind since he figured out which end of his digestive tract smelled worse but, for some reason, it never occurred to him to complain about his being shaken down by a con artist relative. This is important because it's why Jean will never confront him with it should she learn the truth.
This is because she remembers who John is and how he has a low tolerance for honest mistakes. If he ever learned that she didn't know who Fiona was and assumed something he didn't feel like understanding, it's rather obvious that he would inflate that one good-faith error into a licence to doubt everything she might tell him. After all, we know that he can't deal with his kids ignorance and mistakes so his thinking "CHEE! She was wrong about Fiona so I don't know if I can trust her about this heavy rain storm washing out the roads" is pretty much what's going to happen. It's not about his being a slow-witted buffoon who can't communicate on a meaningful level, it's about people who can't read his alleged mind.