Threads is using ActivityPub solely because it was available and stable to build with.

That's it. It's just the fastest springboard this garbage company had to get yet another social media platform out the door when they sensed blood in the water.

Edit: This is a response to people who think FB users will somehow discover the fediverse and be converted by this action. They won't.

Or that FB has noble goals in choosing AP or will somehow be forced to be better. They won't.

It's just good software.