Exactly correct John :) Well actually (thinking about it again with the head I had on when writing the article);

The effect of pumping a little energy back into the utilities grid, is to drive down the grid generation capacity. Effectively, the domestic power meter goes backwards, subtracting previously consumed energy, making it look from the grid like less energy was consumed. The money paid to the consumer just subtracts from the money paid by the consumer historically, though at a lower rate of course, the grid will not pay as much per unit, as the customer paid per unit in the first place. But the net effect is apparent reduction of generating capacity.

I believe the issue would be fixed, if the energies from each source are signed mathematically.

Positive from solar, since solar adds to joules put use on Earth, and negative for extracted energy, since extracted energy has the net effect of removing Joules put to use on Earth.