It’s not misleading.
The one gallon comparison in this case, is that the batteries will propel the car the same distance as one gallon of gasoline.
As someone else pointed out, the batteries actually hold no where near the energy of a gallon of gasoline.
Volt battery pack ~17 kwhz.
1 gallon of gas ~ 36 kwhz.
If anything, the author is being kind to the battery pack here.
The 17 kwhz number being half of the 36 would explain why consumer reports only got 28 miles instead of 56 on a full charge. Since the electric motor is much more efficient at converting stored [electrical] energy to mechanical energy, it would be expected to go much further on a 36 KWh charge than a ICE car could go on a gallon of gas.
The “little” misleading part of this article, as I said, is that it’s implying that a gallon of gas in an average car should have the equal mileage potential as the same amount of electrically stored energy in an electric car’s battery. You don’t need as much stored electrical energy to go the distance that the same amount of stored chemical energy in gasoline can propel you.