You've set up an unfalsifiable belief there. No matter what evidence is brought up to the contrary, you can just blow it off by claiming said people never reveal the truth to whoever is arguing with you. It's a fallacious way to shut other people down and that is, kind of, a little bit dickish.
I don't think that's the case here, though. The parent admitted that they fell into the exact same trap: assuming that certain conversations didn't go on because they weren't a part of those conversations, but later learning that wasn't the case, and it was their attitude that kept them out of those conversations.
It's of course not universally true that's the case for everyone, but I think it's worth thinking about. If your attitude is dismissive of someone's lived experience, it's not likely that they're going to go out of their way to include you in conversations about it; on the contrary, I'd expect them to explicitly exclude you in order to protect themselves.
