Better yet, read them! They answer your question.
>The existence of two books endorsing a point of view is not evidence that this point of view is mainstream.
Since "mainstream" is a judgement call, I don't think anyone can directly demonstrate that to your liking. If you're genuinely curious about this subject, this would be a good time to do some reading.
What question is that? I don't see how reading these books could possibly persuade me of anything other than there are two people who espouse some overly extreme ideas with regards to racism. And I will happily concede that without reading them. So I was wrong when I said that "absolutely no one" believes this crap. Two people believe it. Maybe even 100. Maybe even 1000.
But McWhorter claims that third-wave anti-racism is mainstream (his word) and so far I have seen zero evidence of that.
If you wouldn't mind reviewing https://news.ycombinator.com/newsguidelines.html and taking the intended spirit of the site more to heart, we'd be grateful.
However, McWhorter makes the claim that third-wave racism is mainstream. The existence of two books endorsing a point of view is not evidence that this point of view is mainstream, or even widespread. If it were, then flat-eartherism would be mainstream.