AskBeehaw

Sure, that's the embrace part of embrace, extend, and extinguish. Get their users to use their platform with all of the pre-existing networks and content, so that it's a viable alternative to Twitter, and they bring such a big community that the major established players want to interact with it.

Step two, declare that they've got some great ideas on how the space can be improved, but activitypub doesn't support them, and they don't want to wait around for the standard to update; we need to bring these improvements to our users now! So they unofficially "extend" the standard, so everyone on Threads is getting the new gee-whiz feature, but they can still talk to everyone else. Meanwhile, everyone else's experience is a little bit worse. And while we're at it, maybe we add something that feels bad for both sides when a non-Threads user interacts with a Threads user. So gosh, I mean, why not go ahead and migrate your account over to Threads? You don't lose anything; you still get to interact with your existing networks, nothing bad happens, just that now you also get the cool features.

Step three (and this one happens much later), the overwhelming majority of users are using Threads, because it does all the cool stuff that Mastodon can't do. So now we've got another great idea, but unfortunately, this one just isn't compatible with activitypub. So, we're gonna give you guys some heads up in advance, if you want to stay connected, you gotta switch over to Threads, and if you don't it's just gonna be a wasteland because all of those users aren't visible to you anymore, and your grandma is gonna be sad that she can't talk to you because you won't take the simple step of signing up for a free account.

This isn't idle speculation. Google did this exact thing with XMPP a decade ago.