The previous discussion on Meta has progressed quite well. Note that codinghorror (founder Jeff Atwood) reacted, and it shows some shortcomings in AP / Fediverse positioning maybe. He writes (emphasis mine):
Activitypub is a good idea, but even brand new software implementing it as first class citizens have to fix quite a few holes in the specs – so for us it makes sense to wait and see.
Also, this is totally doable as a plugin if it is something a particular group is super passionate about.
These ‘holes’ mentioned there are arguably fixed by the additional range of (standardized) technology, apart from AP, needed to plug them. They are WebFinger, HTTP Signatures, OAuth2, etc.
The fact that this isn’t immediately clear is due to the fact that no website, including activitypub.rocks, informs good enough on the topic.