> it is written by somebody who purports to understand happiness for a sizable fraction of people.
Why does that seem strange? There is a large body of research into human happiness. I haven't looked into it closely, but I would be surprised if it completely failed to generalise. It seems completely reasonable to me that someone could suggest "if you act like this rather than like that, you'll be happier" and be right about a majority of people.
See for example: http://lesswrong.com/lw/bq0/be_happier/
(This is a seperate question from whether the author of this particular article knows what he's talking about. I do not take a position on that.)
Also, it is written by somebody who purports to understand happiness for a sizable fraction of people.
At least he causes happiness in a large number of people who laugh heartily at such a self-centered approach.