Note that your source above states: "Dimpling occurs more easily with a correctly adjusted bearing than with a loose one that rattles and clunks. Rattling replenishes lubricant between balls and races, something that would otherwise occur less easily."
Seems reasonable to me that a bearing with just a bit of preload would still allow a little movement of the balls under the vibration of a straight ride on a rough surface while one with enough preload to make it feel stiff (as in the OP's case) would be more likely to have the lubricant squeezed out at the contact point. And that's been my experience in practice - the only times I've had a problem develop was with headsets adjusted a bit tighter than usual.