[gtc]

I would take issue with the Discount Tire guy's last sentence. It is not the lugnuts that support the weight of the vehicle.
Torquing the lug nuts generates a clamping force beween the face of the wheel and the hub.
The force applied, the interface area, and the coefficient of friction for the two materials will determine the static friction. This must be high enough to support both the corner weight of the vehicle, and the torsional forces from the brakes and/or engine.

A good example of inadequate friction in a joint like this is the 911 CV joints which use 8mm bolts. If the face of your stub axle or CV joint is not clean and dry, the CV joint will slip and put the six bolts in shear stress, and they will eventually loosen and/or fail.