My theory is that he is affected by America’s racial reckoning, and is blaming social media for it. So he has come up with these convoluted explanations to justify his insecurities.
The quote you complain about obviously can't be about racial diversity. He introduces a problem with diversity, and then links it to problems which, he says, and emphasizes in TFA, have increased very sharply since 2010-14. But racial diversity has been rising steadily for decades with no inflection point near 2014. Furthermore, he goes on to explain how social media leads to a fracturing of cliques and a tendency to maintain parallel lives. It's clear from this context that the "diversity" he's talking about must be a sort of diversity of perspectives and not of heredity or anything else like that.
Not ominous, in mid 2020 the whole of America realized the country wasn’t treating millions of its citizens fairly and started adopting policies to improve equity and diversity, and that upset Professor Haidt. Here he was among the protestors when a school district tried to enroll more minority students in a program.
