A single ended output only has a ground, no differential output. My diagram acknowledges that the balanced cable is supposed to be a differential (though, a lot of "balanced" amps do not actually have a differential output), but can be just as easily tied to a ground. The headphones don't supply any voltage, so you don't have to worry about tying their grounds together, because that's exactly what happens when you use a single ended cable with a single ended output. Also, that diagram clearly shows the negative half of the two channels tied together in a three pole TRS plug, which is weird. I can't understand any of the writing, so maybe this is saying that's not how it should be done, which is definitely true.