I think it’s more that the Apple Watch doesn’t support Spotify than the other way around. Apple prioritizes its own music service for the watch.
I believe for a year or so the Watch OS has supported the APIs Spotify would need to do offline playback, but they have not yet used them for whatever reason.
Spotifys accusation: https://timetoplayfair.com/the-case/
Response from Apple: https://www.apple.com/newsroom/2019/03/addressing-spotifys-c...
HN Discussion: https://news.ycombinator.com/item?id=19377322
Since, watchOS 6 - that's pretty much not true
https://9to5mac.com/2020/04/29/pandora-shows-spotify-how-to-...
I'm surprised to hear about the lack of usage though - Apple Watch support is the only reason I'm using Overcast.