HomePod doesn't show in default audio output for Apple TV

It seems that option is now reserved for full size HomePods with Apple TV 4K, however you should be able to AirPlay to HomePod mini. See Stream content using AirPlay with Apple TV - Apple Support. It isn't immediately obvious, but you need to select the AirPlay icon for the Apple TV in order to see additional speakers that could be selected.

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Also check if they are available in the list of audio devices in the ATV Control Panel (press and hold the Home/TV button). However this does not make them the default audio output.