I know they're quite different technically. But practically, what does ActivityPub unlock that was not previously possible with RSS and basic web tech stack?
I think I have an idea of the answer. RSS may provide a way for users to "subscribe" to content from a feed, equivalent of following and putting it in a unified feed.
But it does not have a way for users to interact with the poster, like comments or likes. This may be possible with a basic web stack though, but either users will have to make accounts on every person's site, or the site has to accept no user auth. (but this could be resolved with a identity provider standard, like disqus does)
I suppose another thing activityPub does is distribute content to multiple servers. Not sure if this is really desirable though?
One more difference is that RSS is polling based, meaning that subscribers have to actively ask every hour or so if thre is new content.
On the other hand, ActivityPub knows who is subscribed and can actively distribute new content to other servers who can in turn send push messages to their users, letting you know about new content within seconds.
So activityPub uses push architecture to push to other servers / instances, but it doesn't push to users does it? I would imagine from instance to user, it is still pull based.
So effectively it's a load distributer thing, I suppose, right?
As far as I know, ActivityPub only applies to server to server communication. Still, many applications that implement ActivityPub (for example Mastodon) do use push notifications for their clients.