I took a look at the link Q posted. The only way that makes sense is if GM is assuming that the cost of electricity is close to free. What they may be overlooking is that you don't charge a battery in an instant, the power is a multiplication of volts times amp which equals watts, and you pay the electric company by the kilowatt hour. A kilowatt is equivalent to about 1.3 horsepower, so a 50 kilowatt electric motor is roughly equal to a 60 horsepower gasoline motor. You shouldn't lean on this too much because electric motors can be made to put out a great deal of low rpm torque, meaning that an electric motor of surprisingly low horsepower can compete with gas motors at low to moderate speeds. So if you are expending 50 horsepower over an hour, you've consumed 50 kilowatt hours of electric 'juice'. A medium price for a kilowatt hour is 20 cents. So you've gone an hour using your 50 kilowatt electric motor , and consumed say 10 bucks of electricity, the cost of replacing that 50 kilowatt hours. If your car will go 60 miles an hour with that motor, you've spent 10 dollars to go 60 miles, How much gasoline will $10. buy? These days in my neck of the woods, you can get 3 gallons. So your equivalent is about 30 miles to gallon, which is good but not great, and far far from the kind of thing GM is talking about.
On the other side, you may be able to go faster on less horsepower, and I think you can shoot for an energy equivalent of 100 miles per gallon, but I'll be very surprised if the Chevy Volt achieves this.

Batteries don't charge for free, they don't charge with perfect efficiency, and they have a major efficiency problem when they are drained too fast. You might be able to drive 40 miles at some stable speed like 40 mph, but if you pass somebody at 60, you might have now reduced your range by HALF.

Anyway, that's the way I look at it. The math is pretty simple.