I think a lot of this can be put into that hefty "Americans are so weird about race" file.
I know race and ethnicity can be complicated. I have Romani ancestry. I don't call myself Roma because I didn't grow up in the community or in touch with its traditions. I don't look stereotypically Roma (sometimes people think I'm Jewish, which I'm not). Very few people know this about me, and if I ever mention it, it's as a curiosity. At most I'd say it gives me an instinctive sympathy for the community, a desire to learn more about it, and a certain saltiness towards non-Roma who go around loudly taking offence on behalf of a community they don't belong to or know anything about.
But I'm not American.
It seems to me there's a particular thing about Americans that, if they're not of recent immigrant background, they're pretty much all mongrels and often know little about their ancestry. That's part of why DNA testing has become such a big deal. I think it's also connected to lots of affluent, educated Americans having abandoned the idea of the melting pot and moved towards a kind of woke segregationism where race is all-important and there's a premium on being able to say that you're not baseline white.
Hence the Americans who pop up on genealogy subreddits saying "23andMe says I'm 3% Italian, can I claim this ethnicity" as if ethnic heritage is Pokemon.
You see it a lot with claims of Native American heritage. Now huge numbers of Americans, especially in the South and in black communities, have some kind of vague family folklore of NA heritage which they've no reason to disbelieve. Sometimes it's true, sometimes it isn't, but if it's just a matter of them having an apocryphal story about a Cherokee GGM then it's not really harming anyone.
But then you look at Native American studies departments in universities and there seems to be quite a trend of academics (it's almost always women, for some weird reason) who've built a career on pretending to be NA when they're really white. They get exposed on a semi-regular basis. I've heard an estimate that, out of the academics working in NA studies, maybe only 20% really are Native. This seems to cause less outrage in universities than insensitive sports mascots or Halloween costumes.
Or you get the case of Buffy Sainte-Marie, and with her it's not just that she's spent 60 years pretending to be something she's not, it's that she's won a slew of awards specifically earmarked for Native musicians, that wouldn't be open to an Italian-American woman who has an affinity for Native Americans.
That's when it starts to look a bit like blokes winning medals in women's sports.