I take issue with the title of the article, which seems to equate positive thinking with naivety. It's link bait for exactly the people who don't need it: the despondent and cynical.
Reasonable self-deception is actually a very important thing for us to develop, because we're not good at doing things we don't believe ourselves to be capable of doing. It's not about thinking you're superman, or pretending you've accomplished something you haven't. It's about giving yourself the go-ahead to try.
However the article is kind of correct.
Positive thinking without action is almost a waste of energy, however positivety leads to optimism which leads to action.
Context: I'm the CEO and developer of positivethinking.net (mobile app)
Frustrating to read articles like this
> This simple process, which my colleagues and I call “mental contrasting,” has produced powerful results in laboratory experiments. When participants have performed mental contrasting with reasonable, potentially attainable wishes, they have come away more energized and achieved better results compared with participants who either positively fantasized or dwelt on the obstacles.
> When participants have performed mental contrasting with wishes that are not reasonable or attainable, they have disengaged more from these wishes. Mental contrasting spurs us on when it makes sense to pursue a wish, and lets us abandon wishes more readily when it doesn't, so that we can go after other, more reasonable ambitions.
Sounds like positive, but realistic, thinking taking into account the obstacles is what's being recommended.