It is very much true in that if you do break something in a truck this size, it is a much bigger problem than say a landcruiser.
however, there is also a little more to it- Say you go overlanding in a landcruiser, with a max weight of 3.2t. the vehicle itself weighs 2.4 from factory, before all the aftermarket fitments- so lets say that the end vehicle, unladen, weighs 2.8-3t. this means you only have at best a quarter tonne of theoretical payload. in fact, i know of several touring cruisers that unladen weigh around 3.3t!
then take my dads MAN, for a typical example of a truck-based overlander.
cab-chassis weighs 4.5t, with our motorhome body (which i might add is much more sturdy than could be afforded on a landcruiser), now weighs unladen around 7.5-8t. it has a GVM (max laden weight) of 10t, so youv got at least 2t to play with, fully loaded i dont think you could fit more than 1.5-2t in the thing (including fuel and water), so the truck is never over-weight, as compared to the manufacturers max weight.
Andrew