@msokolov @gregprice I am not an expert (so perhaps @evan will hop in and correct me), but my understanding of ActivityPub so far is there is effectively no "fetch" mechanism at the post level at all.
ActivityPub (Server to Server) is a "push" protocol, with the only "fetch" operation being "a list of things directly posted by an Actor" (ie, account); there's no per-post fetch endpoint at all.
Comparing to email: I can email you, you can fetch my _sent_ box, but threads are not available.