Hello @ascot,
The reason you are now seeing the Sonos One in the Airplay menu is because of the recent update that released Airplay 2 support for certain Sonos products. At this time, Roon does not support Airplay 2 playback. While most Airplay 2 products are backwards compatible with the original Airplay protocol, some devices like the Sonos One only implement Airplay 2 support.
-John