Folly in this case can mean mistake. "The folly of leadership is knowing that no matter what you do, behind your back there's hundreds certain that their own solution is the sounder one". Will mean A mistake in the part of leadership was believing that whatever you do, others will have ideas and believe that those ideas are better. Another example example: the man's folly was driving on wet winding roads where he believed that no other car drove. The man made an action without thinking of the consequences.