He must have had his highest cowboy boots on. Per this article, more like 5'7" or 5''8" ;
"The head of a major motion-picture studio, shortish, guesses that Redford is five-ten. An average height investment banker who has met Redford estimates five-nine—“with his cowboy boots on.” And to a towering West Coast television producer I spoke with, Redford looks to be five-seven or five-eight"