I've been just playing around with ways to post or host stuff on Nostr and that NIP was me making an RSS based music playlist and posting that to nostr. I guess a nostr app could grab it and make that...
It shouldn't break any existing apps as those RSS tags on Nostr events are just new information other clients will have to adopt. Actually it probably make sense to have links to RSS XML feeds itself as it's a protocol level concern. Podcast index just happens to be a resolvers. So we can still keep feedGuid and itemGuid to make it easier to explore it.