The overproduction of PhDs

I believe that he was referring to the fact that individuals who are what American academia terms "underrepresented minorities" will have an easier time finding academic employment than individuals who are members of other groups.

For some time there's been very little hiring for tenure-track positions in American universities. Only a small number of jobs open up each year, typically due to retirements of existing faculty. And many universities are very determined to increase the percentages of particular underrepresented minorities in their teaching ranks. So they will be inclined to keep the position open until a suitable applicant of the right race appears.

So if a PhD student happens to be a member of a group that's already well represented in the teaching ranks, and doesn't happen to be an up-and-coming research star, that individual might discover that there are very few opportunities out there.

It's something that potential PhD students who are thinking about full-time academic employment probably need to think about. It's certainly relevant to the subject of this thread.

My own anecdotal observations (I have no citations) is that the phenomenon is probably more prevalent in the humanities than in the hard sciences.