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’ve been digging around for similar, but haven’t had much luck. I posted a few conversations I found on the topic in !lemmyfederation@lemmy.ml
Same here