They've said the max torque of the EV is 1000+ ft/lbs. Based on similar number from Rivian, we can guess on the order of 800hp. The gas generator will not produce anywhere near that kind of power.
Pretty sure this is just because it's a prototype. There's a video of the pickup tailgate where that trim isn't even attached at all - just a loose piece that the guy had to put in place. The final trim will almost certainly be automatic just like in a Rivian.
One configuration gets you ~350 miles and the other gets you ~500. Five hundred is more than 350 so it will be more expensive regardless of the cost to manufacture it.