Every existing ActivityPub application requires you to rely on a server operated by someone else that controls your identity. This application puts you fundamentally in control of your content: it is just served from your own device by default, with the option to have another server pin your content (all of your content has Dat addresses).

Also, this sort of "use this, not that" dismissal ignores the fact that almost every decentralized application can be bridged together. It would not be hard to have Mastodon/Pleroma pull someones Unwalled.Garden feed and put it on its federated timeline, or conversely for Mastodon/Pleroma to expose its users feeds as Unwalled.Garden files.

Okay, but why RSS over Activity Streams?

It's not actually RSS, it just uses a very similar model of static files shared off websites.