It was just a swag I came up with on a proverbial napkin some time ago, based on disc loading and FM, limited by rotor radius due to minumum hub spacing for high speed forward flight.
I have no doubt you could increase the GW and manufacture larger heavier rotors, however the requisite disc spacing increase sees exponentially diminishing returns in Vmax due to drag.
Also, consider the XH-59A/S-69 tipped the scales at 9,000 lbs and one of the primary goals of its unfunded successor XH-59B was a "significant weight reduction", which ended up eventually yielding a 5,300 lb X2 TD.