That's kinda what I'm wondering.
I was under the impression that gasoline was used to maintain the vehicle's speed and to charge the battery, that the battery & electric motor took over for acceleration and the brakes aided the battery recharge. Having the car donate electricity to the grid only seems to make sense if the car is spontaneously generating power somehow.
As far as I know (I'm certainly no expert) what they're talking about is not science, but science-fiction.
Alternatively, if you want to sell electricity to the grid you could do it w/ a generator. I often wondered, on the porch of my family's beach house, how much money we're wasting by not putting a wind generator on one of the lots...
I seem to recall discussions in the past about running excess power from home sources back into the grid -- as I recall, the downside was it cut into the power company's profits. I may be misremembering . . .