He doesn't mean that the thinner spokes are going to permanently stretch. Rather, that they stretch more than thicker spokes for a given tension or force.

The benefits of thin-gauge (in the center) butted spokes like Revolutions is partly weight, and partly it's that they stretch so much more than spokes with 14g or 15g center section (Revolutions are 14g at the ends, 17g in the center section). A spoke that stretches more can handle greater amount of rim deflection without losing tension. However, too much stretch is a problem for drive-side rear spokes (which are already high-tension and are not in danger of losing tension under all but the worst rim deflection). Too-thin-and-stretchable spokes on the drive-side is a problem specifically in the trailing spokes, because under heavy pedaling torque, more stretch in the trailing spokes (which transmit pedaling torque to the rim) can cause the rim to shift over and rub the brake pads.

If you want to go light-as-possible but avoid this problem, you could use 14/15/14 spokes on the drive-side trialing, and 14/17/14 spokes on the drive-side leading. Although this may present its own problems in the wheelbuild process, it should end up with a wheel that's slightly lighter and just as durable as a wheel with all 14/15/14 on the drive-side.