[I thought] 2021 Apple TV HD is the same as the previous model Apple TV 4K (5th generation), they just changed the name. The 4K models all perform the same in Zwift, only the older 4th gen is inferior.
Edit: Wiki suggests they discontinued the older 4K model, which would mean yours is the same as the 4th gen unit. Best bet is to look at the model number on the bottom and look it up, what matters is the SoC. I would cancel the HD and buy a 4K model to be safe.
Final edit: yeah, the 2021 Apple TV HD has the A8 SoC which should be avoided. Sorry for the confusion!