He shot that because the conditions were completely different.
He also shot that because he hit all but 1 fairway and hit all 18 greens in regulation, with most of those being within like 20' or something like that.
It was a totally different course 50+ years ago. In addition to being significantly shorter, it had a bunch of trees too(OAKmont). Over 15,000 trees have been removed from the course. While the rough was challenging in 1973, the rough has been grown out and thickened at Oakmont for each of the subsequent Opens(not that it would have mattered much, since Miller hit all but 1 fairway).
Johnny Miller is an unbearable 17nut, but he legit played lights out that day, tee to green to hole. He is like a vegan and a crossfitter combined though. Holy hell will he let you know what he did that day.