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.
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.