they're fundamentally different things. Activitypub is a protocol for multidirectional sharing of content, RSS is for unidirectional publisher/consumer distribution. there's no reason an ActivityPub instance couldn't also provide RSS feeds of their content.
I built this: https://sr.ht/~rakoo/rss2ap/. It posts RSS entries to a given account, and that account can now be subscribed to, entries can be commented, etc... It works well for one or two feeds but can get tedious if you want to follow many feeds and not spend too much time on that.
