I'm a person, not a prop to be used by a corporation to score social points.
All the evidence in this story points to the conclusion that these particular people do not feel so used.
Considering this was done consciously what is message we are meant to come away with?
Is it
a) to show the world that black women can be engineers at successful companies?
b) to show the world that companies can be successful with black women engineers?
c) to show the world that companies are more successful with black women engineers?
In each of those cases what are the implications of them not feeling like a prop?
d) to show the world that companies are more successful when they don't adopt practices that exclude black women engineers?
Then, to be fully charitable, replace "black women" with "statistically underrepresented people", or whatever term you feel comes closest to that which will allow us not to pointlessly litigate.
Then, I think the answer is (a, b, d).
Finally, the simplest, most obvious implication of them not feeling "like a prop" is that they share the objective that their employer has in promoting the idea. The term "prop" is derisive. We don't feel like "props" when we work with other people towards shared goals, even though our work is, directly or indirectly, helping those other people as well.
I suppose that's not exactly a charitable assumption or provable.
But I'm generally happy when the interests of companies line up with what I believe to be the interests of society as a whole.
Companies that go out of their way to be environmentally friendly are most often doing that in order to seize a marketing niche. That's a signal sent with a profit motive. But that doesn't bother me; why should it?
Like you, I do remain wary of the motives of companies when they send signals about social values. Their actions might not match the signals they're sending, or, having firmly established those social values as part of their "brand", they may cease serving those values altogether. I think it's good to be wary of companies.
But I think we should be careful about about letting the social values themselves take flak due solely to that wariness. The ulterior motives of Slack are a small issue. Whichever way you feel about it, diversity is a big issue. Let's not lose the forest for the shrubs.
I suspect we may not entirely agree about the merits of the particular social values ("diversity", whatever) at stake here. I believe the underrepresentation of women and African Americans in tech is a major problem. But either way: you seem like a pretty reasonable person, so, thank you for being that way.