The meaning of, "who would pick someone who smells like fish and mud? who would pick an unpopular tax collector? who would pick leaders from filthy wharves and toil-filled fields? but he did, and together they changed the world," is

The answer is: The effective leader multiplies accomplishment through delegation

Effective leaders would have the awareness that they could never achieved success without the collective help of their team members.

Because of this, most of the time they need to learn to trust other people and delegate some of the power to them.

To do this, the leaders need to realize that they sometimes need to rely on the people that have different expertise and characteristics with them. (which is why the references on different type of people form different backgrounds exist in the quote)