He is correct. The leaf is the part of the flower that has the more potent effect. It may not be as well known for some reason recently, but throughout history that was the part of the flower specifically used for its diuretic effect and as far as I know it's the only one that was studied and succeeded in displaying that effect. Not saying that the root may not have some effect but the leaf is the more potent of the two (not that it is a crazy amount anyway).