It was lean4. In fact he has made lean4 versions of all of the proofs in his Analysis I textbook available here

https://github.com/teorth/analysis

He also has blogged about how he uses lean for his research.

Edit to add: Looking at that repo, one thing I like (but others may find infuriating idk) is that where in the text he leaves certain proofs as exercises for the reader, in the repo he turns those into “sorry”s, so you can fork the repo and have a go at proving those things in lean yourself.

If you have some proposition which you need to use as the basis of further work but you haven’t completed a formal proof of yet, in lean, you can just state the proposition with the proof being “sorry”. Lean will then proceed as though that proposition had been proved except that it will give you a warning saying that you have a sorry. For something to be proved in lean you have to have it done without any “sorry”s. https://lean-lang.org/doc/reference/latest/Tactic-Proofs/Tac...