The author is saying, among other things, that the editors who decided that the person was not notable did not have the domain-specific knowledge needed to enable them to do the research to make that determination.
Wikipedia does not want to be a primary source. That's what Sullivan seems to have overlooked about why subject matter experts' opinions are not eagerly included—they haven't yet been vetted by any third party. Notability is defended by bringing citations, not complaints.
The author is wrong. If he were right, the point would be irrelevant.
One of the users who ended up recommending deletion, DGG, is a professional librarian who makes a habit of rescuing biographies from the rubbish bin. The domain here isn't search engine marketing: it's biographies. Plenty of Wikipedians understand that domain, and understand especially well Wikipedia's criteria for inclusion about biographies.
But his point is irrelevant because Wikipedia can't require topic expertise for participation. They have no way of verifying it, and to try ends up with a totally different beast. (For people who have tried the domain-expertise approach, check out Citizendium.) Wikipedia has to be maintainable by non-experts, because that's 99% of the people who want to volunteer significant time to make a free encyclopedia.
