We added passkeys to existing password authentication for a python flask app, and it took about 40 hours. 20 hours to get basic registration/authentication working, and another 20 hours to tweak the UX (things like popping up a "Want to use passkeys?" dialog occasionally, a passkeys page so users can manage them - all huge problems according to the article).
So please don't put passkeys in the too-hard basket after reading this article. Users love them, and they are going to be ubiquitous very soon.