for this pic, i would suggest lowering the gamma say to about 0.4 to really bring out the darkness of the clouds. bumping up saturation to the max (255) would also give some colour to the clouds and help to maintain the greens of the leaves.
i think what he means is that the tree and the sky are both fighting for space and attention in this photo. it may be better to isolate the tree, but let it be surrounded by lots of negative space in the dramatic sky. that way the pic will look less cluttered and will have more impact.