That one is new to me. In 50 years of pursuing the high end, most of the time my amps weighed well over 100 pounds and went on the floor.
If you are lucky enough to have mono blocks they definitely sound best placed within a foot of their speakers with ultra short speaker cables. Virtually all amp stands are low to the ground intending to be within a couple inches of the floor.
I could see someone say that because they tend to be the heat generating component. But not anyone familiar with the high end audio industry.