He was the one calling it a center torsen diff, I know it is a transfer case.
The limited slip system is more commonly found in the front/rear differentials so that the slip is managed between wheels. If the LS were located in the transfer case it could only control power to front vs rear and not between the driver and passenger wheels as is needed when turning.
When switching the system into 4Hi with no locker installed in the front/rear differential all that is happening is the the computer is telling the transfer case to lock in and grab the front driveshaft so that it is getting power from the drivetrain rather than free spinning while the car is being propelled along by the rear axle entirely. 4Hi is perfectly safe (mechanically speaking) to use on pavement at normal road speeds if there is no locker being activated in the axle differentials because each wheel still can spin at its own rate independent of the opposite side.
The drive shafts are turning at whatever rate the transfer case is geared to relative to the output shaft from the transmission. The transfer cases only job is to convert that output shaft power into forwards and rearwards paths for the driveshafts. The driveshafts will not be turning at different speeds, the axle shafts inside the axle will be, but this is managed by the differential and so that is a moot point.
The point is that if the manual says to drive 10 miles in 4WD then he can do that, there is no reason this has to be on gravel mechanically speaking... but if you want to follow the manual to the letter than go for it.
Lets Cover a few things:
Never use 4Lo on pavement or any speeds over ~25. 4Hi is fine to use being as though the axles are both being powered, but the wheels are still free to spin at different speeds for turning at normal operating speeds.
Limited Slip differentials will automatically lock the two wheels together in what ever axle has the limited slip system so that all power coming into that differential is equally diverted to both wheels. This aides when one tire is in the air or has no traction, but the other wheel on that axle does.
Locking differentials, be them electronic or air, will lock the two wheels so that they both get equal power regardless of traction condition (never drive on road at normal speeds with differentials locked)