I was simply pointing out that HDMI ARC is not the only way to get audio from the TV to the AVR. Nor is it necessarily the best. In most cases, the optical and ARC outputs are identical, although with a few TVs, optical supports 5.1 at times when ARC does not. Optical is also not subject to the same oddities that can plague ARC due to the way HDMI CEC works (or doesn't work).
I am not saying either interface supports 5.1 when the source is the TV's USB input. I don't know one way or another.