Well, one of the two cited references have diversity in their article several times. I don't know how to sift through the BLS reference.
Is there any indication that sentence needs a citation more than other claims and assertions? Is there something about the idea that an overly homogenous workplace leads to (or attracts) people with less decency and politeness?
Generally the article follows the "summary - supporting details" format, which is pretty standard in papers like this. You are being extraordinarily generous to just let them slide on this one
"the leading elements contributing to toxic cultures include failure to promote diversity, equity, and inclusion"
and yet there is no other mention of "diversity" anywhere in the article, and no surveyed person mentioned it.