Well the posts to inbox are generally for incoming info. Yes, there’s endpoints for fetching objects. But, they don’t work for indexing, at least not on mbin/kbin. If you have a link, you can use activitypub to traverse upwards from that object to the root post. But you cannot iterate down to child comments from any point.

The purpose is that say I receive an “event” from your instance. You click like on a post I don’t have on my instance. Then the like event has a link to the object for that on activitypub. If I fetch that object it will have a link to the comment, if I fetch the comment it will have the comment it was in reply to, or the post. It’s not intended to be used to backfill.

So they do it the old fashioned way, traversing the human side links. Which is essentially what I lock down with the managed challenge. And this is all on the free tier too.