It's more accurate to say that they are thought-experiments. They exist to mirror what people might think or feel when faced with beings that either have never existed, except in the imagination, or beings that might be proven to be real entities at some future point in time.
Within the fictional worlds where aliens and intelligent androids are real, they can serve as markers of diegetical diversity. In that fictional world the rights of androids or aliens to fair treatment under the law are as "real" as anything else.
What the fantasy-beings don't have is extra-diegetical diversity; aliens and androids and super-powered mutants cannot "stand in" for POC because the former don't to our best knowledge exist. But that's not the same as saying that the attitudes toward diversity within the context of the science-fictional story are irrelevant, simply because those attitudes are predicated on thought experiments. If that were true, then the whole of fantastic literature would be completely irrelevant when compared to the whole of representational literature. Not that there aren't people who believe just that, but it's surprising to encounter even loosely similar convictions on a board devoted largely to superhero comic books.
I've noticed, however, that often detractors of "mutant diversity" only make their arguments one-way. I'm familiar with one online jackass who loudly complained that "the X-Men are not an allegory for racial tolerance." He wanted to take away whatever "credit" that fans might have allotted to Lee and Kirby and Claremont et al on the basis that mutants did represent a form of diversity. In the place of that credit, he wanted a "debit" to show that the X-Men actually represented the Evil White Social Order. So it's OK to consider fictional thought experiments as having relevance to diversity, as long as it's a negative relevance.