As someone who's used both, the way federation works on Bluesky/Atproto is very different to how ActivityPub works. It's very much a backend thing, letting that do the heavy lifting and prevent the user from worrying about it as much as you have to here, rather than trying to make its fediverse a whole experience.

As for Tumblr, there was talk about bringing it to the fediverse and Wordpress themselves have already allowed users of their hosted blog service to opt-in to this, but have not done it for Tumblr. And Threads? Well, there was a pact to fediblock Threads because of its ties to Facebook, so there's that.