The Head of Instagram commented on this:

“We’re committed to building support for ActivityPub, the protocol behind Mastodon, into this app. We weren’t able to finish it for launch given a number of complications that come along with a decentralized network, but it’s coming,” he said.

“If you’re wondering why this matters, here’s a reason: you may one day end up leaving Threads, or, hopefully not, end up de-platformed. If that ever happens, you should be able to take your audience with you to another server. Being open can enable that.”

Look, I don't want to be Devil's advocate for Meta, but I'm kinda hopeful as I truly believe they entering the fediverse can be a good thing in the long run.