He meant to write cooling; the moderate (warmer) parts leave, leaving the fanatical 'cold' members. I agree that 'fanatical=cold/moderate=warm' seems a little backwards, but it's that way and not the other because it's an analogy to a real physical process, while evaporative warming doesn't exist.
From 2007: https://www.lesswrong.com/posts/ZQG9cwKbct2LtmL3p/evaporativ...
https://en.wikipedia.org/wiki/Evaporative_cooling_(atomic_ph...