This track routing trick actually works! - had to try it myself. NO OUT-OF-PHASE issues, no plugin (but he does do a plugin comparison toward the later part of his tutorial. I wonder what it'd sound like with TWO different guitar tones panned a bit doing featuring the same trick?

 

"it's actually a danish letter..."

What you see here is very likely plugins reacting differently depending on the phase of the input. It doesn't work always, it really depends on what you have on the channel, and if your DAW puts the pahse invert button at the input or output.

Nice trick though.

 

This "trick" only works because he uses some effect plugins on the tracks that produce different output everytime. You can see "Renaissance A..." and PRS Archon M..." in his mixer. By inverting the phase of one track you get the difference of the effects. This is what he calls "broken speaker effect", and this would be silence without the effect plugins.
So this "trick" wouldn't work at all without using effect plugins that modulate the signal in any way.

 

Well, not really without a plugin. When you duplicate a wave only, phase invert it, pan one hard left, the other hard right and mono it, they cancel out each other perfectly. That's just the nature of things.

Nonetheless, his trick, which isn't a routing trick at all, works surprisingly well.