Well, I suppose saying a rock could have coached them is a bit of hyperbole...but it might not be that much.
Consider that this was a team that won 67 games and the championship the year before. This team returned TWELVE players including all of the top 9 in minutes from the championship team. That's a total of 92% of the floor time from the champs returning. And those 12 guys played 95% of the minutes in the season Walton coached. He was flat out handed a championship team with effectively no turnover who had already played with each other for a full year (at least).
I can just imagine myself, not at all a basketball coach, being thrown into that interim situation. I'd say, "OK guys, you know what you're doing. Now go out there and do what you've been doing and win the game!", and I'd never ever draw up a play (because I can't), and I'd try to manage minutes and substitutions as best I could. Even if I led that team to lose every game that Walton got them to win by 6 or fewer, they're still a .721 team.
The job that the Warriors did with Walton as an interim coach was indeed impressive. I just don't know how much of it can be attributed to Walton.