If an advert is hardcoded by the author into the file they upload, how do you suppose Spotify should remove that? They don't have rights to do that but even if they could, they would need to programmatically detect when the ad starts, ends and cut it out from the file? Soft Skills Engineering does this for example, they record their own audio for the ad and play it like it's a part of the podcast, I don't mind that as it helps authors, but that has nothing to do with Spotify.

I've heard about some dynamic adds Spotify supposedly adds, that are usually not related to the podcast itself, but I've never encountered a single one like that.