Well, that US definition is actually
broader than the Australian/English one. Notice it says:
> Laws prohibiting national origin discrimination make it illegal to discriminate because of a person's birthplace, ancestry, culture, or language. This means people cannot be denied equal opportunity because they or their family are from another country...
The parts I've italicised are exactly the definition of "national origin" discrimination in Australian and English law. From what you are quoting, the US definition includes the Australian/English one, but also goes beyond it.
> I find it highly unlikely that any court is going to find it discriminatory if you spell it "color" instead of "colour"
Courts don't just decide whether something is or isn't discrimination–they decide whether it is illegal discrimination. Not everything that meets the bare legal definition of "discrimination" (on some prohibited ground) is illegal – something may meet that definition, but nonetheless still be legal, because it is justifiable, or the harm it causes is insufficiently serious, or it is permitted or required by some other law.
I doubt any Court would uphold a discrimination case based solely on English spelling differences – but that's not because spelling is per se outside the definitional scope of "national origin discrimination" – obviously it is an aspect of the "culture, or language" part of the definition you cited. Rather, I expect they'd rule that even though it is discrimination, it is legally permissible discrimination, either justifiable by the unreasonable expense of doing otherwise, and/or because the modicum of harm caused by it is too trivial to merit intervention by the legal system.
But what counts as illegal discrimination is not really relevant here, because OpenAI has spent a lot of resources on trying to remove certain subtle biases in their AI models, which arguably goes well beyond what they are legally required to do. I'm criticising OpenAI's decisions, but I'm not claiming they ought to be prosecuted or sued over them. I'm simply citing legal definitions as informative as to what words mean, even in a context which goes beyond the strictly legal.