How to use Siri remote to turn off TV using IR

I have an older TV that supports HDMI-CEC for power ON but not for power OFF of the TV.

So my new Apple TV Siri remote works fine to turn on the TV, but I'm unable to use the remote to turn it off - requiring a second remote just for this function.

It seems that the Siri remote can learn how to do Volume Up/Down from other remotes using IR; why can't we add the ability to do the same with Power On/Off?