She is talking about the Ultium platform (think Silverado EV), not the HummerEV specifically. With the HummerEV aerodynamics I expect right around 350 miles of EPA range (less on a straight freeway run @65 mph, and much less if you drive 75-85 mph), assuming the battery is 200 kWh. If the battery has more capacity, more range is possible, but I do not think GM massively understated their range on the HummerEV.
Realistically, because you do not get 0-100% effective use typically, if I am planning a highway road trip, I figure I will be charging every 200 miles, dropping to about 100 miles with a trailer. I think in the Hummer Ev your trip will go faster if you drive slower, but that will be hard to do in the Hummer Ev, with such an awesome looking truck it will be hard to putt putt in the slow lane, so plan extra charging stops. Not much different then earlier Hummers stopping for gas, they did not go far on a tank of fuel.