Spotify doesn't currently support HomePod, which is really annoying since the HomePod Mini looks awesome.
You can vote for HomePod support on Spotify's community site:
https://community.spotify.com/t5/Live-Ideas/iOS-Implement-Na...
You can vote for HomePod support on Spotify's community site:
https://community.spotify.com/t5/Live-Ideas/iOS-Implement-Na...
Spotify support would mean you can ask Siri to play a song from Spotify and it works directly, without using a Spotify app on a second device.