Good explanation. I did the following experiment. Starting with the same photo, I varied the black point from 0 to 224 in increments of 32 using (a) Levels and curves ("RGB") (b) LCH editor ("Master lightness").
(a) Levels and curves
(b) LCH editor
As you can see in the second filmstrip, pushing the black point in LCH is like dimming the lights; the subject gets darker but the colors don't change. With Levels and curves, the colors become increasingly saturated as the scene darkens. I think this is what Jason Odell means when he says using the LCH editor "preserves color."