The author never assumed that most people who voted in the election watched the debate. There could have been a tiny number of people who watched the debate; the author simply thinks those people may have been biased toward Lopez.

It’s not clear what live viewer reactions have to do with a survey of people who saw the debate on television. In any case, (B) is consistent with the author’s theory, since the survey respondents’ opinion about who won the debate still can be due to bias.

This compares the likelihood of voting for Tanner among viewers and nonviewers. But what matters is whether the viewers were more likely to vote for Tanner or Lopez. (C) could just mean 1% of viewers were likely to vote for Tanner (as long as that is greater than for nonviewers).

This is strong evidence most viewers were not biased in favor of Lopez before the debate, which suggests bias was not the reason viewers of the debate said Lopez had better arguments.

The narrow nature of the victory doesn’t change the fact Lopez won. The author never suggested that the victory was dominant or that the specific magnitude of the win affects the likelihood that viewers of the debate were biased.