What's interesting is that the author have a data oriented strategy to find a good definition of a leader: he processed a lot of data to find which teams had really exceptionnal success IN THE HISTORY OF ALL SPORTS.
When he found the 8 teams with the most exceptionnal success, he looked for what they share. He found that all of those exceptionnal team success coincide with the arrival and departure of a captain.
Then he looked for shared trait between all those captain. What he found is the best definition of a leader.