No, I would not say it is true of many great HWs. Most of the great hw's you mentioned may have been around Usyk's size or smaller, but they still fought other people around a similar size - which Usyk already did at CW.
Usyk then fought guys who are much bigger than him. Most of those guys were not undersized for their era at HW, Usyk objectively is. Weatherspoon is 240, Joshua is 240, Dubois is 250 pounds, Chisora is 250, Fury is 280.
Every opponent Usyk fought was much bigger than him. AND he he beat guys around his size, which was the golden age of his era. If you want to say guys like Ali/Frazier/Norton could have been CWs too, then sure - we'll call them the unofficial golden age of CW then.
The point still would remain the same. Usyk still beat many elite opponents, and looking only at his HW career makes little sense, any way you slice it. Even if what you said was 100% correct, it still would mean that Usyk's sample size of high level opponents is high when you include his CW career.