Still, while both lenses are valid, I'm focusing on the consequences lens, because we're discussing an Atlantic article that tries to invalidate it. It's not misleading to call the memo "anti-diversity", if you're focusing on the memo's role as a social artifact rather than as a dissertation, and that's a valid perspective. Words often serve both roles, and it's important to consider both.
(Incidentally, I don't find the memo's argument to be especially sound, either, so it's not just that it was expressed carelessly—but that's sorta beyond the scope of this thread.)
if I had to summarize the memo with an "anti-..." prefix then I would say it's against artificially skewing the gender makeup of the workforce via preferential treatment of female applicants/employees over the discriminated male applicants/employees.
if that's what "diversity" means then I don't know on what logical basis you would be defending the "let's make sure we have a 50/50 gender makeup of the workforce even though the proportion in the applicant's pool is nowhere near that" position.
I myself (and from what I've read in that "manifesto" I believe this is author's position as well) welcome diversity - the 'IT sausage fest' is totally a downside - but not at the expense of ppl getting gender-discriminated.