Honestly, I believe it's the last reason. Levelling the playing field would be relatively easy: blind auditions (easy in some fields, like music, more difficult in ours — but doable). I'm really not convinced that diversity of thought has all the benefits we attribute to it. At the end of the day, I think it's fashion: we want to be seen as fashionable, and diversity is in fashion.
Regarding diversity of thought, her example is a great one: 'And look, if you put a gun to my head and asked me, given absolutely identical abilities to do the job, whether I should hire a woman who came from an affluent background, aced her SATs because of access to a stellar prep program and supportive parents, went to a top school and interned at a top tech company over a man who dropped out of high school and worked a bunch of odd-jobs and taught himself to code and had the grit to end up with the requisite skills… I’ll take the man.' I'd take the woman, not because she's a woman, but because she almost certainly has more of the requisite skills than the man in the situation. If they actually have identical relevant skills, then maybe he'd be more interesting, just because he's so unusual, but judging by appearances the woman is more likely to have the skills I'd need.