I used to do re-enactment in the UK, and after almost every show I'd have some idiot wander up and say "I'm a Saxon!" and blather shite at me about what that meant about their identity and culture.
If I asked if that meant they weren't American, then obviously they'd react in horror at the suggestion.
The idea that my culture, my history, can just be co-opted as part of someone else's cosplay identity, is tiresome at best. But then they walk up to me and expect me to recognise them as a fellow Saxon? No. Fuck off you annoying fucking wanker.
And I notice that none of them claim to be English or even British. Oh no, too much Braveheart and The Patriot for that.
It doesn't keep me up at night, but I think it's tacky and vulgar.
Or, to put it another way: your criticism is tacky and vulgar. Perhaps what you're describing is "cosplaying" but that's not how immigrant communities see themselves. I do in fact know the perecentages of my national makeup but pizza and beer aren't how I celebrate that. Nobles know their ancestry down to the smallest detail, is somebody really tacky for knowing that technically they are 1/4th Italian? I don't think attacking somebody's identity is ever fair; it costs you nothing but is everything to them.