I just tried this remote....and it only is partial "compatible".
Volume +, -, Mute do NOT work at all.... to control the Apple TV 4K system volume.
The Power button, does not work at all. Does not trigger the Apple TV to sleep or pwr back on.
After further frustrating back and forth with Channel master support, it appears this remotes Volume and Pwr buttons are separate/independent IR control of a TV or soundbar, NOT the Apple TV it self.
If you use a bluetooth audio device, headphones, speakers etc, or AirPlay speakers, then you need system volume control of the Apple TV.
This is how I use the Apple TV, via Bluetooth DAC to speakers.
I also occasionally AirPlay to 2x HomePod minis for late night quite tv watching.
It is not made clear of this limitation and that the Volume and Pwr buttons do not actually work with Apple TV. If they claim it to be a Apple TV replacement remote, then it should do the most basic functions of turning it on/off and adjust volume.
If you do have the "traditional" setup of a TV/sound bar, The way to program the separate IR control of TV is terrible, they literally tell you to press the button "up to or more than 100 times until your TV responds)
EDIT: So, Channel Master actually CALLED me. (you input your phone number when u submit a support request.)
The guy knew exactly what I was trying to do, as he uses AirPlay speakers as well, and confirmed that this remote can not control Pwr or volume via IR to the Apple TV. It is designed as a independent IR control of soundbar/TV.
He stated that they do not even have in their system Pwr or Volume IR codes for the Apple TV direct control to emulate the Apple Remote.
It may be that the Apple TV remote uses its Bluetooth to control those system functions.
Either way, if you use Bluetooth or AirPlay for audio, you will need to use the Apple remote to adjust the volume.