I should have been more clear: they do on IOS, when airplaying to Airplay 2 enabled speakers, because of this (from. ars technica, see below).
You simply select all the speakers that you want from the IOS Airplay menu (not from within the Spotify app itself).
Users can still use the original version of AirPlay from within Spotify. And if they're using an Apple device to run Spotify, they can use OS-level AirPlay 2 support to gain access to some of the potential benefits of AirPlay 2 with Spotify's web, mobile, or desktop apps.