While this is true, it doesn't preclude one side being completely unhinged from reality.

In a scenario where there is a normal side and a stupid side, both sides will also feel that the other side is stupid. So this means absolutely nothing.

Well, there aren't 2 opposing sides, and that feeling doesn't change the fact that some beliefs are indeed stupid.

Well yeah, I think their beliefs are stupid. It would not surprise me at all that they hold other stupid beliefs.

Like, I believe flat earth beliefs are stupid. Do people that hold them believe my beliefs in that area are stupid? maybe, but I'm pretty comfortable with that.