You're right. RSS builds on top of the web. ATProto does not. I'd say RSS is a resource format / content type, not a communications protocol. A resource format intended for syndicating updates – exactly what ATProto and ActivityPub do (but decided to invent new formats instead of extending RSS/Atom. JSON all the things!).

It is very much about identity. To use tangled you need to use ATProto and authenticate using ATProto – rather than using the existing open standard for authentication used by pretty much everyone at this point (missed opportunity to login to Tangled using GitHub). What's crazy is people still use the web to interact with tangled anyway.