Threads users didn't sign up, it used their Instagram account. Even if they used it once, they click across, and boom, account there and they cannot delete that Threads account after. An initial day registration does not mean a MAU. It's like saying MySpace is huge now because everyone had an account...
Edit: Usage plumetted massively- https://www.theguardian.com/technology/2023/aug/14/threads-app-slump-daily-active-users-twitter-competition