This statement is incorrect. A touring bike sees significant torsion loads, above and beyond those experienced by an unloaded bike, due to leverage exerted by panniers mounted at a distance from the bicycle's centerline. More spokes, and to a lesser extent the spoking pattern, affect a wheel's lateral stiffness much more than they do its vertical stiffness. If the main load on a touring bike wheel were vertical, you could tour quite happily on Ksyriums.

I built my touring bike wheels myself: 26", 36 hole, 4-cross. Is it any stronger than 3-cross? Probably a small amount. But if I ever break a spoke, there's a good chance I'm going to have to loosen one or two more to get the broken one out. Things wouldn't be quite as tight with 700c 36h, but 4-cross is probably best left for 40h or greater wheels