that's mostly because metis despite being mixed people are considered separate/unique from either whites or amerindians.
Huh, it's almost as if ethnicities are solely defined by humans, and are therefore subjectively determined.
Enough for your hypothetical inbreeding issues to have surface over that time, particularly with the lack of the technologies and record keeping needed to avoid it. Anyway, going by
I'm pretty sure that many technologically astute Martian ethnostates would also pursue genetic engineering in less than a millennium, and that'd circumvent the inbreeding issue altogether if properly applied.
that may well be true but it wasn't said they'd be selected solely by ethnicity.
The only other selection factor in the thought experiment was a willingness to join the colony.
EDIT: Personally, I wouldn't be morally opposed to a bunch of Martian ethnicists or racists living in their own colonies without negatively affecting people.
They certainly can't become a country and claim territory, however, due to international treaties, but if their territory-free society is based on voluntary principles, have at it. I'm just concerned for the viability of their society and lineages, because they're all walking on shaky philosophical ground.
For example, what happens if the majority labels themselves as a new ethnicity or race, regardless of whether they've diverged due to genetic drift or manipulation? What if the number of new ethnicities and races created like this grows exponentially over time?