I don't know if Bill or Edge put in the limit, I would imagine that they tested the load limits in actual vehicles by loading them up and seeing how the engine responded. That or they just guessed
... If you load the truck up with that much weight you shouldn't see a problem, and maybe even a little more than that, but I wouldn't push it too far. What are you carrying that weighs so much?
Also I wonder if there is a difference between towing and in truck weight... it might make a difference where the weight is located... but that doesn't make much sense to me so I don't see it really making a difference.