Apple TV HD. How do I connect a stereo pair of HomePod Mimis? 'Default speaker option' is missing

Hi Paul.

Yes, the default speaker selection is only for AppleTV 4k.

Set up home theater audio with HomePod mini or HomePod and Apple TV 4K - Apple Support

I don't know why. It may have to do with the processor, other chipset, radio standard but that is just a guess on my part.

Unfortunately I don't think the chances are good that AppleTV HD will support this feature in the future. The device release date was, I believe, 2016. The hardware's age is likely a very limiting factor.