It falls under the fallacy of half truths. It is a selective thinking whose main objective is to focus on pieces of evidence that hold to be true to the believers and omits some facts to present an accurate argument.
The diction of Steinbeck here in apparently describing the dustbowl conditions of the Dirty Thirties is speaking of "tenant men" or presumably men who were tenant farmers perhaps who were allowed to live on the land in return for working it and that they "scuffed" their way home indicates that the dust was so thick they had to scuff but also perhaps that since they could barely make a living under the poor agricultural conditions they did not walk confidently but scuffed.
Answer: Sean is not an effective speaker because he is disrespectful of others.
Although passion and decisiveness are important when fighting for a cause you care about, being disrespectful is not a good quality. When someone uses forceful language or interrupts, he is not letting others listen to counterclaims. This is a big problem as it limits the exchange of ideas. Moreover, someone who is disrespectful can be intimidating, which reduces everybody's participation in the debate.
