RSS is a loosely specified document format with many implementation variants that basically represent a list of items with links, titles and a few other fields.
ActivityPub is a W3C protocol and API specification for basically a generic federated social network with client server, and server to server API calls to create and manage activities and activity streams (a related specification).`
It's not about being 'better' but about being something different. Representing an activity feed as an RSS feed is easy. Mastodon in fact has support for RSS. So, you don't even need to use mastodon to follow mastodon users and their micro blogs. For example, this is how you get Neil Gaiman's RSS feed
ActivityPub is a W3C protocol and API specification for basically a generic federated social network with client server, and server to server API calls to create and manage activities and activity streams (a related specification).`
It's not about being 'better' but about being something different. Representing an activity feed as an RSS feed is easy. Mastodon in fact has support for RSS. So, you don't even need to use mastodon to follow mastodon users and their micro blogs. For example, this is how you get Neil Gaiman's RSS feed