" It doesn’t work I think because if you connect everything to a home theater receiver or separates they only play one source at a time…"
Yes, that’s correct.
He does it by sending the audio to a device that is different from the device controlling the TV. Like sending audio to an endpoint/amp, and using the above linked method to send it to a tv that is complete unconnected.
Another way to do this is to use the appletv app .