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To summarize the replies to my question, so that you don't have to continue incorrecting each other in this thread:
Author of ActivityPub spec: "Yeah that's not how any of this is supposed to work."
Author or Pixelfed: "There are some reasons, they're not great, we hope to fix that someday."
A bunch of people:
Literally everyone else: "The way it was implemented is Right and Perfect, better things are Not Possible, you must not Understand Software."