You're just declaring it. I see why you do, it comes from a specific Enlightenment view of rationality that's quite appealing. But it clearly doesn't hold up in all situations. At the bare minimum it doesn't hold up in the expert community of historians who we should trust as the epistemological authorities on the subject.
The reason is fairly complicated, but the simplest way to put it is getting back to what I've brought up time and time again. We aren't making claims about certainty, because we can never go back to the past and find out for certain what happened, and even if we did we'd still be using a lot of interpretation to understand it.
History is about making arguments that are more or less likely, more or less convincing. It's a different but, contra positivists, not lesser understanding of validity. The sort of validity you're suggesting simply doesn't make philosophical sense in history, which is why the positivist program in history fell apart, and, to a lesser extent, why my field has rapidly decline in popularity since the 80s.
Well that's not sufficient. If they posited a meaningful counter argument, the expert community at least thinks the issue is contentious, and they are ignored, which is exactly what happens in these thread, then that bit of information is certainly relevant, though quite not particularly nuanced in this instance, to the argument at hand. Regardless of who is right and wrong here, the popular narrative in the US is wrong. As I'm sure you know, experts that agree with the sentiment make rather different arguments. The core issue here is that most arguments, admittedly on both sides, are essentially substanceless. They are nebulous claims that no one is really backing up in a meaningful way.
He can't really address the argument head on, because both sides are simply presenting contradictory narratives without much in the way of justification. The only thing there is to address is why you picked the unjustified argument you picked.
Regardless, if you want a strong epistemological, but that's going to be different that you're expecting I think, argument on the topic you have to go to experts. No one in this thread is an expert, and people in this thread don't seem particularly interested in actually looking at what experts have to say.
I'd argue if someone is making the argument that their arguments exist totally outside of their nationality, education, sexuality, or spiritual beliefs is also incredibly dumb.
The statement in your post isn't nuanced, but it's closer to the truth. Most people do not believe the things they believe for particularly philosophically sound reasons. It's probably kind of rude to dismiss large swaths of people based on their nationality, but it's definitely really stupid, and blatantly ahistorical, to ignore the impact our historical situation has on our thoughts. Even at the most obvious level you have issues like language that are undeniably in play.
Okay so this is fair enough, but then we're right back to the fact that most of the positions people are taking are pretty substanceless.
We can discuss those things, but we should never delude ourselves into thinking we're talking about them as dinge an sich. That's a lesson the discipline has really taken to heart in the last 30 years.
Here's a genuine question for you. Why do so many Americans believe the popular narrative?
Obviously that wasn't the best way to go about doing what he was doing, but I honestly don't see how you dismissing his argument, especially when he went back and explained it in greater detail, is that different.
When intellectual historians of the future look at how we were talking about this issue what sorts of things do you think they will be saying?