That is true and a good use case for fail2ban. Useless was probably a strong word, what I really meant was of limited utility in increasing the security of the SSH service.
The main reason I use fail2ban is I got tired of the log file noise/bloat. I use key-only access on my servers already, with the key stored on a hardware token (Yubikey).
I guess the question then is why you're looking at failed Auth logs. Failed auths are boring, doubly so on a key only server. Successful auths are where the fun is at.
When I first set up fail2ban it was because I got annoyed that the machine on my desk was making regular "clunk...clunk...clunk" noises from the hard disk as it wrote another failed-auth attempt to the log every second or so...