So Roy wasn't wasting his breath?

No, because for once he was right.

Actually, Roy's crop was more subtle. He didn't shorten the top right clump, as you have done here, but made a slight crop through it, to trick the eye into thinking that the tree might reach yet greater height. In other words, he left a little bit to the imagination.

If the whole height of the tree isn't to be included (which e.g. Constable always included) then the whole composition would be different, with far less North.

No, what he's saying is to include the whole tree, but just shave the teeniest bit off to create an impression of greater height.

Outstanding shot and ambiance!