As far as i know the Spotify Mediaplayer only shows targets which support spotify connect like the Amazon Echo Speakers. For Sonos, try using the sonos media player only and select spotify there as source.
For example, if a spotify playlist is running on one speaker, i would like to change the current speaker to another and this should resume with the playlist of the other one
Strangely i cant select spotify as the source for the sonos component. Any idea?