"Nothing’s scarier than getting going too fast down a hill on a skateboard." Actually if you know how to power slide, going down hills is actually quite fun.
Also, is this a full electric board, or do you have the option of pushing and then you can use the motor when you want to?
Since the motors are engaged full time (no freewheel) and are self-exciting, they're going to give some drag due to iron losses, which is magnified by the gearing of the belts, as well as some stored angular momentum.
So I think it could work as a non-powered board, but not very well.
I haven't ridden it, but I know it's got regenerative brakes so it's safe to ride downhill. Not sure how they're activated. Based on your position on the board perhaps?
Also, is this a full electric board, or do you have the option of pushing and then you can use the motor when you want to?