I don't think he has the end of the taper in mind. Rather, what I believe he's saying is that if 20' (or 30') of 6wt line will load your 6wt. rod, switching to a 5wt line on the 6wt rod may require 30'(or 40') outside the tip to achieve the same loading. He seems to believe that the lighter, and thus thinner line will go through the wind better.