Juicy Studio: WCAG and the Myth of Accessibility ... still digesting this. It's a topic I've seen/considered before. It's a very tough one; but my overall thought is that people with cognitive disabilities probably require a different kind of support. I'm not convinced that web developers can cater to literally everyone - the cognitive disabilities issue really crosses over to a content issue, rather than a technical issue.
At any rate, I'd hardly call accessibility a "myth" just because it's not perfect yet. Not so long ago, nobody appeared to give a shit about anyone other than fully mobile, fully sighted users. Mr Leitch would do well to reign in his high-horse tone.