People don't have to be stupid to be manipulated in the way the author suspected; it just takes being busy, having a specific goal that one wants to realize (buy software) and focusing on it, and not being particularly interested in the internal implementation details of the mechanism of manipulation.
As marketeers become more sophisticated, and choose to employ more devious methods of swaying opinion - the ability of the general population to successfully decode these messages improves.
Take a look at some advertising messages from the 50s, or some propaganda posters from the first or second world war - their messages wouldn't hit home today (unless they're served together with a liberal dose of irony).
Audiences become more sophisticated over time.
The answers to the above questions are, respectively: "A lot," "A huge number" and "Less than 8%, according to a Google survey."
You could replace a good 80% of the instances of "stupid" with "busy" in design discussions, and you'd end up with better discussions.
BTW - I love your ASCII art logo design.