For people that come straight to the comments – Threads was also what Instagram called a standalone messaging app that they introduced in 2019 and never took off. They shut it down at the end of 2021, which is what this article references.
Apparently they've re-designed it a bit (or started over from scratch) to turn it into a Twitter competitor now that Twitter is a bit... turbulent, and are relaunching in a few days (which is currently being discussed on HN here[1]).
Might've been better to just start with a fresh name rather than resuscitating a brand that has already failed once, but what do I know?