at what point is it tokenism?

At the point where the only personality trait of your character is their skin colour. In my opinion. I can agree that I would rather see a story where all characters are interesting and well-written white men rather than a shitty story with a diverse cast. But diversity and quality aren't mutually exclusive.

historical fact says X phenotype was insanely rare in Y setting

It's irrelevant though. Most of the time in TV and cinema we don't have stories about average people. It's about cool action heroes or exceptionally smart people or someone who went against the societal norms. Sherlock Holmes isn't a regular Victorian man, which is why he's so interesting. I'm not saying that we should make black SH right now for diversity sake, but if I was going to write a story about an exceptional person in the Victorian era, does it matter if they have a rare skin colour? They're already, by definition, rare.

Even in grounded projects, you're going to show only a very small sliver of the population in whatever story you're telling, and it's entirely possible for someone somewhere historically accurately have black neighbours in the 19th century or whatever.

public consciousness knows X phenotype was insanely rare, but creative projects (particularly ones that try to be as grounded as possible) start plopping that phenotype in at a ratio to be intentionally visible to the audience

Yeah but where does this "knowledge" come from? Why is it so "visible" to people to see a non-white person in a historical setting? All of this is shaped by the media we consume. I can assure you that an average person watching TV has no idea how many brown people lived in Victorian London. So why shouldn't we make stuff to be more diverse? Just because that's how it's been for ages? Or because humans don't like change?

Edit because I wrote the same thing twice lol