Respectfully, I don't even know if I would agree with that on a number of levels. Even the existence of one example should have stopped you in your tracks already, because that's all it takes to demonstrate as a principle that bias doesn't work that way. But I also think those are just the tip of the iceberg, and there are broad swaths of widely believed claims that are fundamental to people's sense of their own political identity also in exactly this unfortunate category.

On the left side of things some (not many but some) there's a lot of apologizing for and minimizing of the horrific human rights records of the U.S.S.R. and China, some degree of falling for present day pro-Russia misinformation, and uncomfortable truths about the party's own closing ranks around people with a history of sexual abuse. And on the right there's a lot of deep-in-the-bones denial of evolution, or historical revisionism about their ties to the "party of Lincoln", or who won the 2020 election, and it's expanding every day to include new topics like prediction of hurricane paths to provably wrong information about nutrition, and so on.

I would say it's so omnipresently pervasive in the present day that it's impossible to wave away and is increasingly fundamental to explaining and understanding polarization.