I think "diversity" is not as firmly a property of groups as "underrepresented" - consider someone remarking "oh hey, now we've got some diversity in this room" when a person of color walks in, or a chair musing that they need more diversity on a committee and asking the department's token woman. Yeah, those are still technically statements about groups, but it's pretty clear which individuals are "diverse".

Or maybe it's just that "diversity" is more common than "underrepresented" and therefore more ripe for appropriation.