Just from looking at the pictures since the Merc is a cabover that would allow a lot more GVWR. I am also guessing it has a larger frame larger axles and larger brakes as well as larger tires. F450's only have 19.5 rims. Towing capacity takes all things into account, braking, frame strength, wheel rating, axle rating, transmission etc etc.

From the looks of that Merc it would be far more comparable to a F650 or F750.