they do?
Siri on the phone can control the Spotify app on the phone, so by default the AirPods can also. that's not Spotify connect.
as of now there's no Spotify "app" on the HomePod to allow local playback, so there's nothing for siri to control.
I know it's not a traditional app, but there must be a process running on the HomePod to allow it.
Spotify connect is where the Spotify app can connect to and control other devices on your network. I have sonos speakers and an echo dot, and when you click the devices screen in the the Spotify app, in addition to the "airplay and bluetooth" option, those speakers are listed. If you select it that way, you control the Spotify player on that remote device, instead of sending audio to if from your phone over airplay.
It's similar to how you can either send audio to the HomePod via airplay, or connect to it, and the phone just acts as a remote for the player in the HomePod.