So, as I understand it based on other systems I’ve used/worked on that had a few similar feature sets, ActivityPub will effectively send out events of “things”. What that is depends on whoever is telling ActivityPub about it. So, the data for a mastadon post (what are these called?) Would likely be pretty different to the data shape of a post on Lemmy. Not specifically the same, but I don’t think there’s anything stopping somebody from developing a server that can handle multiple types of content. Kbin has a similar thing between threads and microblogging. Two content types supported there.