That is a good point. The essay was written about how DNA testing would change the mind-killing aspect of politics, but we probably should consider what effect it would have on just plain politics. Probably, if a lawyer came to the local political party and said that he was interested in running for some local regional office, and her DNA analysis said that she was likely to be a sociopath, the party would be less inclined to support him. Hopefully, DNA testing might make the quality of the candidates that reach higher levels of office a bit higher. There would probably be some benefits, but I doubt that it would help the mind-killer.
I wrote an article for h+ predicting that the rapid fall in the cost of gene sequencing will allow U.S. voters to learn much about presidential candidates' DNA. The candidates won't be able to stop this because:
DNA analysis has a decent chance of reducing political bias by providing objective information about candidates. If, for example, 70% of the variation in human intelligence is determined by identified genes then DNA analysis would reduce disagreements among informed voters over a candidate's intelligence.