These bikes are now winning MX races. ICE can't compete. And they are competing with the major brands at price point. 6 hours is definitely competitive with the big boy MXers which hold a little more than a gallon. It's a race, not a marathon.
Of course these bikes are not for us geriatric individuals, that isn't the point ! Not sure about you, but I ride off road, and my limit is closer to 2 or 3 hours
The point is that this is the beginning of change. It's a great reason for wishing to be a teenager again! If I had the money, I'd look into them. They have modes that increase range, make them less wildly powerful.
And, you would be amazed at how easy it is to ride a KTM off road. They are ultra lightweight, stiff, have "adequate" adjustable suspension, and they are a serious contrast to the low priced bikes we discuss and work on here. My KTM 250SX is incredible. I can lift the front end from a stop, and drop it down in the opposite direction, and it only cost ~$8300 in 2020. And it is made to handle that type of front wheel sideways drop! I got a way better deal, but it is definitely my favorite toy now.