you can easily load a selection from a black and white image
One can easily load a Selection of the Composite Channel (by command-clicking it in the Layers Panel) – or am I misunderstanding You?
I think he has his terminology confused, and is referring to transparency.
But then it doesn't make sense to adjust levels on the transparency channel very often (if ever).
And you can already target an alpha channel and use adjustments on it....
So, I'm not sure what he meant.