Not being able to start a podcast from the Spotify Sonos service is a Spotify issue.  Please vote for the idea in the Spotify community.  If it gets enough votes, Spotify may fix the issue.

https://community.spotify.com/t5/Live-Ideas/Support-podcasts-in-Sonos/idi-p/5642007

Note:  You can cast from within Spotify to Sonos speakers, but the playback controls do not work consistently.  For example, skipping forward 15 secs may skip to next episode.  This is a consistent problem with how Spotify works when casting to any system, not just to Sonos, and the Spotify proposed solutions may or may not work or may work and then reappear.