I think he meant to write heating. Because the moderate (slow) parts leave, keeping the 'hot' members.

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...

Distillation may be a more appropriate analogy. The unwanted part evaporates away, leaving the more concentrated solution behind.

In distillation, the whole input liquid is evaporated by heating, then condensed by cooling and collected.

That depends on whether you want the more volatile or less volatile liquid at the end.