In LessWrong: A Community for Intellectual Progress w/ Oliver Habryka and Ben Pace at The Stoa the two answer to a question about what about postrationalism and metarationalism.
In it they assert that David Chapman's critique isn't direct at the rationality at Lesswrong and that David Chapman hasn't read LessWrong. Given that at the time of this writing David Chapman having 220 karma at LessWrong, I don't think it's fair to say that he doesn't understand LessWrong.
As I understand Oliver, from his perspective rationality at LessWrong is at the moment a pre-paradigmatic field. That means it's a field without a paradigm and we don't have a paradigm of rationality yet. From David Chapman's perspective Eliezer Yudkowsky proposed a paradigm of Bayesianism in the sequences. David Chapman wrote a critique of that paradigms based on his work as a MIT Phd is artificial intelligence in 2015.
From David Chapman perspective some people adopt that paradigm after reading the sequences. Being at the stage of Kegan 4 means being able to reason according to a paradigm. When David Chapman uses the word rationalism he means both people who picked up a paradigm of rationality after reading the sequences and also people who got a paradigm of reasoning from a good university education and are able to reason according to it.
When In the talk Ben speaks about there was a time where he actually did reason from one paradigm and couldn't really engage with other schools of thought. Later Ben moved on and did actually become able to engage with other schools of thoughts. This moving on is likely what David Chapman would model as the transition from Kegan 4 to Kegan 5.
One perspective would be to say that when Ben read the sequences at 13, he adopted a suboptimal paradigm and later moved on from that paradigm. From the perspective of Kegan's framework, adopting that paradigm was however likely very good for Ben's development as it allowed him to go from Kegan 3 to Kegan 4 which is an important step in development. Not everyone moves from Kegan 3 to Kegan 4 and many people need a good university education to make the transition. Making that step at 13 is fast cognitive development.
Globally, we have a problem that while universities used to be good at getting people to make the Kegan 3 to Kegan 4 transition postmodern thought reduced the ability of university programs to get their students to make that transition. At Kegan 3 people have the issue that "other people are experienced as sources of internal validation, orientation, or authority". A lot of toxic culture war dynamics come from people waging power who haven't made it to Kegan 4.
If people who self educate or are in a failing university program can read the sequencing and HPMOR and move from Kegan 3 to Kegan 4 by adopting a paradigm of rationality that's great. It's a lot better then telling them: "We are pre-paradigmatic at the moment, so sorry we don't have a paradigm to give you to make your Kegan 3 to Kegan 4 transition".
Taking that stance has a similar issue as the problem that the postmodernists who attacked paradigms from the perspective of Kegan 5 had. You can't bring people directly from 3 to 5, so a lot of the new generation stayed stuck on Kegan 3 with all the bad consequences that this entails. The people who put postrationalism on their bannar have no good way of dealing with people at Kegan 3 and face the risk of getting filled with people at that level.