More like hardcourts are not real surface, tennis used to be played on clay and grass. Imagine Nadal in an era where USO played on clay. Noserer would have zero US Opens then
Bjorn Borg won 6 times at RG, but won 0 US Open even through it was played on Har Tru (different clay) for 3 years. Borg even beat Connors on Har Tru in 1976 at the US Open; Connors would never have won against Borg at RG.
Your assertion is not necessarily true.