This is almost certainly wrong. The U.S. Open had a partial best-of-three format for men's matches from 1975 through 1978. This includes all three years on Har-Tru clay and the first year on hard courts at Flushing Meadows. In '75, '76, and '78, the men's matches were best-of-three for the first three rounds. In '77, the men's matches were best-of-three for the first four rounds.
The champions in those years were Orantes, Connors, Vilas, and Connors again. The runners-up were Connors, Borg, Connors again, and Borg again. I have never, not once, heard anyone claim that these slam accomplishments should be downgraded in any way due to the tournament's format. (Of course, some online pundits are simply ignorant of these facts, but that's their problem.) No arguments about "inferior" titles. No over-the-top theses about how to calculate any purported reduction in legacy value.
On a related front, no one (to my knowledge) insists that slams won in the pre-tiebreak era are worth more because they often required longer sets to be won.This applies to both men and women. Margaret Court beat BJK in the 1970 Wimbledon final, 14-12 11-9. Is that title worth more than Wimbledon titles won with sets that included tiebreaks? No. No one believes that. The 1970 final may be deemed a great match by many due to its length and closeness, but the championship obtained is not more valuable than later editions.