What the Sheldon Brown's web site is saying about front wheel braking is absolutely correct, if overly theoretical. For any specific rider position (shifted backwards from the saddle or not) the braking force generated by the front wheel alone at its ideal maximum braking force point (as described below) will always be greater than the sum of front and rear braking forces at any other (non-ideal) point. (Assuming, of course, that the wheel-to-road traction is sufficient to achieve that braking force).

For each given rider position, maximum braking force is achieved with front wheel braking only. And it is achieved when the the rear wheel bears 0% of the bike weight, i.e. at that exact point when the rear wheel makes that "infinitely light" contact with the ground. Of course, in this configuration the rear wheel has no traction at all. I.e. its is either about to lift from the ground (if pushed forward/up) or it is about to skid (if pushed sideways). In such situation there's no difference whatsoever between the root reason for these two scenarios.

The only issue with this sort of front wheel braking is that is only possible under the ideal braking force, continuously maintained by the front wheel brakes. Applying excessive force will ultimately result in front flip (or at least front wheel stand), while applying insufficient force will keep some weight (and traction) on the back wheel, taking it away from the front wheel. No cyclist is ever able to maintain this force perfectly and continuously. An experienced cyclist will be able to stay very very close to the ideal by observing the feedback from the bike and adjusting the braking force accordingly.

This inevitably will result in short periods (or long periods, depending on how good the cyclist is) when the rear wheel receives some portion of the bike weight. For this reason it might make sense to apply rear brakes as well, to "pick up" those portions of traction that from time to time "escape" to the rear wheel. Or it might not, since it can lead to rear wheel lock up.