"If I wrote an essay riddled with technical errors staking a position about technology" - this is not really an apt comparison, because politics is something that everyone is meant to be qualified to comment on, by virtue of the fact that every adult is considered to be informed enough about politics to vote. So it's the directionality of the argument that matters, not every fine detail. And given that the real topic of the essay seems not to be politics but leadership, as a successful company founder the author is very qualified to comment on that, and his mistaken beliefs about political details are not central to the argument.
In the case of the particular error you point out, sure it's technically incorrect of him to refer to "the city and county of San Francisco she controls", but as a Democratic Party congressperson representing that area, she could easily influence policy to improve the running of that city/county. So he's not completely off target with that critique.
"The belief that everyone is equally bad/corrupt is wrong and it just makes you a mark"
I don't think that's what he's saying, and it's not what I said or what I believe.
As for “pretending to be wise”; he doesn’t seem to be offering a neutral position and by virtue of writing a very long, reputation-risking post on the topic, he’s not avoiding investing resources, so that post doesn’t seem to be relevant.
But this notion that is morally incumbent for everyone to endorse and vote for a major party candidate is, in my observation, usually an attempt to guilt/shame people into supporting the proponent's preferred party.
How is it not acceptable for an educated person to adopt such a view as: "this whole circus is a stupid game orchestrated by media behemoths, financial institutions, military contractors and other cronies and rent-seekers, and it serves mostly to divide and exploit ordinary individuals and families in order that a small number of elite insiders can profit, and I choose not to play a role in perpetuating it"?
I'm not saying you have to believe that; clearly you don't, and nor do I, completely. But I know of people, educated, objectively intelligent people, not just fringe conspiracy-theorists, who hold this kind of position very sincerely, and whether it's completely valid or not, I don't see how it's an unquestionably wrong or immoral position to hold.