The thing that's different with this proposal is that it's specified to be a one-shot notification. If RSS is a channel/topic than "let me know" is a rendezvous. You could build it on top of RSS (or ActivityPub, XMPP, webhooks, ...).
Whether that is a protocol or an application running over a protocol is semantics, either interpretation is valid.
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.