Answer:
The anecdote within his essay of the preaching slave adds to his argument, it does not detract.
This example is the baseline and inspiration of Twain's argument. Although he does not fully agree, it is where his idea was imprinted which adds to the argument that all opinions are unoriginal and inspired.
Twain carries the idea that men are made up of society's thoughts and opinions. Twain feels that everyone will accept public opinions only for the satisfaction of one's "self approval". Twain expresses to the reader that we all in a sense of sacrifice our own uniqueness to be accepted by society. Corn Pone Opinions are a group of belief with which we each join as an impulsive mode to fit in, as Twain says " the instinct that moves to conformity did the work. It is in our nature to conform; it is a force which not many can successfully resist; what is it's seat, the inborn requirements of self approval"
Hope this helps.