Protocols are normally built on top of other protocols. RSS could be the transport for "let me know." (Though my recommendation would be ActivityPub, XMPP, or email.)

Whether that is a protocol or an application running over a protocol is semantics, either interpretation is valid.

No need for a separate protocol here. You can just read an rss feed until you get a hit on your filter and then stop reading it. Same data over the wire, just a different client.

Any bog standard RSS app should work here.

RSS, literally and with no modifications, solves the use-case outlined in the article. The URL is just a specific one-shot feed, and TFA's request is just a request for such a convention of one-shot feeds.