It is a commonly held thought that harder reeds are thicker. In fact, within a brand/model, all the strengths are the same thickness because they are cut on the same machinery. The different strengths are the result of variations in the cane, and each finished reed is gauged to determine what strength it is. This is why they typically can play so differently out of the same box, because of the spread within the grade. Obviously if he was talking about different brands/models, the reeds actually could be thicker or thinner in the critical tip area. However, I don't know if any of this has any effect on how much mouthpiece one takes. If he was serious about it, it could just be something that he learned to do that helped him in his style/sound.