The meter has been accurate for maintenance since they started putting them on tractors decades ago.
If you put the engine to heavy service, it needs service sooner. If you put it to light service, it is not as stressed, and will not need service as soon. What they are saying is that 2 hours of light duty work has the same affect on the pistons, rings, bearing, oil ect as 1 hour of hard work.
As a comparison, my truck manual specs out changing the oil at 7500 miles if use is light. For medium work, they recommend 5000 miles between changes. For heavy work use(towing, heavy hauling), the recomend even sooner. So, effectively, 7500 miles empty is roughly the equivilant of 3500 miles of hard work. Just a different method of measuring.