the strength is tooting a reply to a peertube video and having a discussion on lemmy in which all these comments are shared
I’m with you. The problem is that this promise is mostly empty, at least at the moment.
ActivityPub, from what I’ve gleaned, is too vague and open ended and under-developed in terms of software for this to be true. The result is that each platform is implementing a sub-set of the protocol and often adding their own custom twists/additions to it. Which means that just because two platforms use ActivityPub does not mean at all that those two platforms can communicate in anyway. And, even if they can communicate, there’s no guarantee at all that this will be usable.
The interaction between lemmy and mastodon is illustrative. Technically they can communicate, and at times this can work well. But the two platforms are hardly mutually enriching each other because the interactions between them are fairly limited in number. And that’s because they don’t talk to each other well. Some of that is because they’ve implemented different parts of the protocol. Some of it is also their differences in design and UI/UX that just add too much friction to consuming and meaningfully interacting with content from the other platform.
What’s more, this problem is fairly predictable and has been criticised as a false promise in the past. At the moment, I’d say it’s fair to say that ActivityPub has not been proven as a way to enable communication between substantially different platforms. That might change over time, though I suspect the load on developers to make that happen will remain high without some major foundational work.
But right now, unless there’s something I don’t know/understand, I don’t see the extra-platform capabilities of ActivityPub playin any role in the success of the fediverse in competition with BlueSky, at least as far as Mastodon is concerned which, as a platform, is relatively happy just doing its own thing.