Spotify is certainly monetizing their podcast platform. At least for the Spotify-produced shows, they’re injecting ads regardless of whether you pay for premium service or not.
Caught me somewhat by surprise one day when I walked past a hospital and got an ad in a podcast for that very hospital (GeoIP for my mobile IPv6 is correct at the zip code level).
In fact, Spotify even allows podcasts to deliver their own MP3s (if you meet the requirements for their "Passthrough" program), which gives distributors that don't want to use Spotify's ad platform more control and is probably a condition for them even listing on Spotify.
By default, and presumably also for old Spotify Connect devices that only support Vorbis, they reencode all files and cache them on their own CDN.