Indeed, it must be one of the compute units available on the newer Apple processors. My iPad Pro 2nd gen also doesn't support it. I really hoope that they upgrade the Apple TV to support spacial audio soon. It really is a nice feature. Also, Mac support would be great. The bigger the screen, the more likely you are watching movies on it.
While they are at it, Apple finally should properly support homepods on macOS. They still aren't shown as pairs - at least on Catalina.
Which has made me seriously wonder if it's not actually about a gyro/accelerometer on the device, but did Apple just design the code to run specifically on certain A chips?
From the support page [1] for spatial audio, from the list of supported mobile devices you can basically deduce that it requires an A10 chip or later.
Which would explain why Intel Macs aren't supported. The only thing it doesn't explain is why the Apple TV 5th gen (4K) doesn't support it, which does have the A10. But I swear I'm holding out hope that a software update this year will bring spatial audio to the M1 Macs and the Apple TV 5th gen. I just for the life of me can't imagine why they wouldn't.
[1] https://support.apple.com/en-us/HT211775