It seems to me the basic ActivityPub specification is written from the perspective of Mastodon and Twitter-like fediverse instances.
I assume Lemmy and kbin did extend this with some more objects or at least agreed how to use the existing objects and activities there to model a link aggregator with comments on top of that.
Is there some sort of specification or design document about this somewhere? All I found when googling were some old links that resulted in a 404 and the current Lemmy documentation seems more focussed on users, admins and developers and less on the protocol side of things.
I don’t know if this is kind of what you’re looking for since I haven’t read it completely yet.
https://socialhub.activitypub.rocks/pub/guide-for-new-activitypub-implementers
There is certainly a lot of interesting information in there but from what I have seen nothing on the threadiverse/lemmy/kbin/… unless it is in the linked forum or other discussion places.