It’s because you can’t “kill” a the AP protocol. XMPP didn’t go away when Messenger and GChat removed support for it, it just went back to how it was before hand, a fraction of tech enthusiasts using it for private communication. It would probably be the same with AP. A separate collection of sites using it to federate information.
… even if Threads abandoned ActivityPub down the line, where we would end up is exactly where we are now. XMPP did not exist on its own outside of nerd circles, while ActivityPub enjoys the support and brand recognition of Mastodon.
Granted this leaves out how Google used it’s influence to control and stagnate the XMPP protocol, but that’s another can of worms.