I would assume there would not be the same application of antitrust law, if any (IANAL) because Spotify isn't a marketplace that sells the podcasts.

That situation would be more comparable to Netflix since you're just paying a bundled subscription and you're getting 3rd-party work as well as Netflix originals. While openness would be awesome to have, they don't really have any obligation to provide it.

Of course it's not exactly like Netflix since you can self-publish onto Spotify so there's wrinkles. But my point is that it's a lot different than operating a buying/selling marketplace.