I wish my electricity was that cheap.
As for the main point itself, I can imagine the intent was to make sure supercharger station are actually available for someone on a long distance trip, but it is quite poorly worded.
That's commercial/industrial pricing. If Solar City is able to obtain status as a generator that can sell power on the open market (combining rooftops into blocks for firm generation using controlled release of power to the grid from sunlight stored in PowerWalls), they'll be able to "sell" power to Tesla at a much lower rate.
As a residential customer in Illinois, my cost is 6cents/kwh, and as low as 1cent/kwh between midnight and 5am.