Laut der Ankündigung arbeiten sie in „Phasen“:

We’re taking a phased approach to Threads’ fediverse integration to ensure we can continue to build responsibly and get valuable feedback from our users and the fediverse community.

Und es ist geplant (irgendwann in naher Zukunft) „fully interoperable“ zu sein:

We will continue to collaborate with developers and policy makers so that people across services have the opportunity to experience the benefits the fediverse offers via a fully interoperable experience, including reaching new audiences and fostering their community.

https://engineering.fb.com/2024/03/21/networking-traffic/threads-has-entered-the-fediverse/