Mr. Sawyer was primarily speaking of blue wing olives, which are fine swimmers. Many of the nymphs that we attempt to imitate are not so good at it. They do their best to reach the surface, or bankside structure, but vary in their swimming abilities. Also, many nymphs are dislodged from the bottom, or are part of natural drift, and are not swimming to the surface. They likely have their legs akimbo, not tucked into their cute little nymphy selves.
Chuck