Airplay is an Apple standard that allows streaming audio/video to different platforms like speakers and TVs from Apple devices. I believe that currently, the iOS app and macOS app of the Spotify client do not support the second version of Airplay which is what the above idea is referring to.
However, my idea is solely focused on supporting Spotify on the HomePod WITHOUT having to use Airplay to connect to the speaker (i.e. natively). This means that one could simply ask Siri on the HomePod to play music from Spotify instead of having to go to my iPhone and manually select the HomePod speaker under "AirPlay or Bluetooth". Additionally, this should allow the HomePods to show up under the "Select a device" menu withouthaving to go through the AirPlay menu.