So I tried a few things just to see what the limits are and confirm what's in the manual:
The only time the truck won't move is if the driver's door is open and the seatbelt isn't buckled - then if you're in drive/reverse (ie backing up a boat or in a tight spot) and you open your door w/o seatbelt buckled, it'll go from Drive>Park. (no ebrake)
With 'Autopark' enabled, it uses the seatbelt to confirm you're in the vehicle and will auto disengage the ebrake. if you don't wear your seatbelt, all you have to do is hit the ebrake button and off you go, no seatbelt required (it'll just ding at you)
I'd say the most annoying part is if you're hopping in/out of the truck at a ramp etc and need to inch around w/the door open; you have to belt up (or just clip the belt in)