The truck couldn't travel at a higher MPH (they already travel at highway speed limits). What they mean is that the truck would get there faster because a human truck driver legally needs to rest for 10 hours every day, but a self-driving truck does not.

The money you'd save is by not having your truck sit idle while the driver slept. You could thus deliver the same amount of freight with fewer trucks. However, trucks would wear out faster due being run for more hours per day.