By using your definition, the Prius is by far the best selling EV on the market.
So no matter what the definition is, it's simply not correct. It's the best selling plug-in hybrid, not the best selling EV.
(Also, Volts still periodically run the engine even if you drive 0 miles on gas, so you haven't been running purely on electrons. You'll eventually need to refill with gas in 100% battery mode).
The distinction is this. A Volt will run and operate normally at all speeds with zero gas in the tank (or the gas tank removed). A Prius will not.
Volt is an EV with a range extender - admittedly, a well integrated and integral range extender.
BMW i3 Rex is also an EV with a range extender - a much less well integrated one, however.
When I had a Volt, I had one YEAR where I used only 3 gallons of gas. Less than a lawn mower. At that rate you are really splitting hairs between the Volt and other BEVS. Try running a Prius for a year, over 10,000 miles, with 3 gallons.
We can also discuss efficiency. The Volt typically runs at 250 wH per mile. With my Model S, I typically run closer to 320 wH per Mile. The Model S uses about 2 kWh (or 24 cents a day more electricity) than My Volt did on weekdays. At that rate, it is about $80 a year or enough to buy about 30 gallons of gas. That is 10 times what I used my most efficient year. Now, if we want to get environmental, how much pollution does it create to generate approx 6000 kWh of electricity. Probably more than the 3 gallons of gas my Volt used. Much more.
Of course, there is much more "hauling a$$" with 416 horsepower than 149 horsepower. But hey, saving the planet is a side job after having a fun drive