If they broadcast them over radio, is it still a podcast? Emailed out the audio files to a mailing list? Distributed them on cassette tape? It seems to me the format is part of it. RSS or something equivalent (Atom), feeding audio files to whatever client you bring, is a podcast. Youtube videos aren't podcasts just because I listen to them in the background (though one might also have a youtube video of one's podcasts) or because I could youtube-dl the audio and listen to it. The format is what gave it the name—if you can't consume it in a podcast client, it's not a podcast. My physical mailbox and my email box both contain pretty similar things, but my email reader can't read my physical mail, because it's not email. If you send me a letter, I don't receive an email, because you didn't send one. If you publish your audio interviews in a closed platform, I can't receive a podcast, because you're not publishing one.
The main thrust of what I'm saying is that RSS is a very very specific part of just the distribution of the actual content. Everything except the RSS and use any app is there on Spotify; the functionality of an open app (download, speeds, RSS feeds etc.), the content style, etc.
Personally I'm not a fan of Spotify locking down podcasts because I do like that I can use whatever app I like but I'm not going to call something not a podcast because it's annoying to get.
