The premise is that the author is a libertarian, except that housing in the neighborhood they live in should be fixed at the price that they can afford, and that the character of the neighborhood from the point that they moved into it should be preserved.

It's only real gentrification when upper-middle class YIMBYs get forced out.

What I said is this is what I struggle with. In general I think too much regulation is what caused this problem. And the question on my mind is, from where we are today, is there a liberalization of the rules that would also help maintain a mixed income neighborhood, rather than the current trajectory (quickly becoming an uber-wealthy country club).

Except that's not what the author argued, at all.

The author acknowledges that a lot of the gentrification is the result of zoning rules, and has only proposed what are primarily less restrictive zoning rules as a potential solution.