The default speaker option is now limited to Apple TV 4K with full size HomePods, but you can still Use Apple TV to play audio throughout your home - Apple Support with other combinations of hardware.
Long press on the TV button on the remote, then click the Apple TV 4K/HD icon on screen to get the AirPlay menu where you can select your audio destination. Faster is to use the Siri button on the remote and say something like "Play on HomePod".
I don't know how long it will stay connected to HomePods once you've done this. I probably use mine more for music than TV, and an earlier setup that woke my Apple TV to display what was playing on my HomePod was probably more annoying than having to make a connection when I want to add HomePod audio to my Apple TV. Of course now that I try it I find my HomePods are still linked after putting the Apple TV to sleep and then trying to use the HomePods for music. Worse it wakes the TV, but won't play anything. I have to dive into the AirPlay settings and disconnect the two first. Grr!
tt2