I agree. Even though the privacy control is opt in to share their posts with the fediverse, I think it's fair for Threads to be counted as a server in the fediverse holding user data. It just has more advanced privacy than others.
Threads barely interchanges with the rest of the Fediverse, it's basically bridged. By this argument you could include Truth Social and Gab, which are directly based on Mastodon source.