Winning it on Sunday and coming from behind while really trouncing the field certainly makes it a more memorable and more impressive feat. To me a 63 is a 63, regardless of whether a course is a par 70, 71or 72. The fact that some of the par 4's at Erin Hills were at or near 500 yards and that the final hole on the final day was 661 yards yet was still among the easiest (maybe even the easiest) suggests that there's virtually no way to compare the difficulty of a course like Oakmont in 1973 versus a modern course like Erin Hills. You'd have to include the vagaries of equipment (persimmon vs. metal, shaft technology, the modern golf ball vs. balata etc.), the physical conditioning of today's golfers and the advances in golf instruction, among many other considerations.
Both rounds were impressive. Johnny Miller for a few years knocked down flagsticks like nobody in his era. His final round to win the Open at Oakmont will be recognized forever as the greatest finishing round for the win of all time. Until somebody beats that record. But if they do, Miller will find some reason to argue that his was better. That's my main beef here, that Miller took time to beat up the course before the tournament started, then tried to belittle Justin Thomas's terrific third round, which included that amazing three-wood finish. That's as close to the final hole Ben Hogan one-iron at Merion that we've seen on a modern golf course in a major in my memory.
Miller shouldn't have dissed it.