The author, Larry Paulson, may or may not like ZF but he has a ton of experience in doing formal proofs in it.
https://isabelle.in.tum.de/website-Isabelle2020/dist/library...
https://isabelle.in.tum.de/website-Isabelle2020/dist/library...
Maybe this is the reason for his dislike. Would like to hear more about Larry Paulson's experience.
(maybe it was no fun at all)