There's also the "DVD phenomenon." Even though I assumed the moment Justin said it that he was making it up, the very fact that I pretty much swallowed the entire series up to S4 without talking to anyone about it, reading fandom stuff, etc, and without any waits between episodes or seasons means that there's no time for little threads of "hmmm, I wonder" to take hold. I had an initial impression and I moved on, and as it turned out, my initial impression was the one that ended up "sticking," if you will, as a matter of Brian's characterization.
But for others, getting the episodes and seasons in dribbles, there's a lot more time, even without fandom input like you describe, for alternative scenarios to play themselves out and different interpretations to surface. I don't think it's a matter of "right vs wrong" on most of those things, but how you saw the show does impact how you interpret it. There's no way it couldn't.
So I wouldn't say you have or had a belief in the unbelievable. I'm not even saying your perception of how Justin said it was influenced by other fans. It's very possible that you'd have felt the same way no matter how you viewed it, because it is, after all, a matter of interpretation. I just wanted to explain that my interpretation of the scene and my embrace of canon are compatible, LOL.