I think it is prevelant in most lines of american thought because, as a nation, they are taught all other nations as hegemonic. Where the u.s is seen as the only none hegemonic nation. It isn't just all brits are white. But all chinese people are han etc.
I mean in 2025 that seems slight ridiculous. Although I think what's more striking is the exceptionalism in the original poster that he still thinks he's right.
It is ridiculous but we see it all the time. The op is a fantastic example. Even being proven that tikka masala is a british dish, they refuse to agree because it goes against what they were taught as ethnicity being a blood and soil thing.