It’s a dull show. It’s not a 1/10 show

Sure, but surely you can see how people overreact when high expectations aren't met. These review numbers are from regular people, the same people who only give 1 or 5 stars on Amazon or whatever. When you see terrible reviews for something from a big name studio, it's because expectations weren't met, not because the content actually sucks.

If Japanese people being prompted by a hater don’t even hate it,

But you're not the audience, westerners are. And there are a lot of westerners who have very strong opinions about Japanese culture. That's the push-back you're seeing, vocal westerners want an immersive experience in Japanese culture, and I think people are thinking that pulling one of the few non-Japanese characters from Japanese history robs that experience.

But as you said, it's not out yet, so opinions here can certainly change significantly once it's actually out.

I think both are examples of legitimate issues fans have with how a franchise is handled. That may not be relevant to you (esp. if you're an outsider to the franchise), but that also doesn't make it "review bombing." I see "review bombing" as a concerted effort to tank a product due to issues unrelated to the product, such as a scandal involving the higher-ups at a dev studio or something.