It’s a small advance, but one that speaks to Meta’s enginerring team paying attention to how the fediverse community is trying to educate Threads users about the possibilities.
The fediverse is now something that you can evangelize about. Its turning into a buzzword ...
The official story is that Meta is worried about being sued by people suddenly seeing their content pushed to some random website without their consent if it's enabled by default, so they won't risk enabling it by default. At least not before the fediverse is huge enough that everything you post going everywhere on the internet is the expected behaviour.
Fair enough really. I wouldn't want to be sued for that either, and they obviously cannot expect Congress to understand.. anything.
Threads is for whoever Meta can sell it to, and I think it was pretty far along in its development before they actually committed to ActivityPub support.