Positive thinking, in my view, is more about saying "What do I need to do make this happen." instead of saying "I will never be able to make this happen." This article seems to use a destructive type of thought that is more like "This will happen to me!"
"Gabriele Oettingen and Doris Mayer asked eighty-three German students to rate the extent to which they “experienced positive thoughts, images, or fantasies on the subject of transition into work life, graduating from university, looking for and finding a job."
I agree that simply imagining things simply working out for you can be damaging. But I think that there's something like positive thinking that doesn't include baseless fantasizing. To think "I get an interview from 1/3 of the applications I send out, and I might be hired for 1/5 of the positions I apply for, so if I make sure I am applying, I will likely find a job" is reasonably positive thinking, but it's more constructive than thinking "Ah, I'm awesome, there's a job I want, and I'm certain they will hire me because I'm thinking positively. I won't bother applying elsewhere, I'm special, they'll need to take me" and then falling into a depression when you don't even get invited for an interview.
On the other hand, both of those situations are better than sitting at home and thinking "I'm never going to get a job anyways, I might as well not apply".
To me positive thinking is about looking at all of the angles, not being afraid to see where things go differently, but still have a positive outlook. "If I do this, there's a 25% chance it will fail. If it fails, I will lose my money, if it succeeds I will make 200 times what I invested. I have enough set aside to try this 10 times. It's worth doing it, and I wont ruin myself if it doesn't work out."
It's that sort of thinking that shows that failure isn't bad, it's just a possibility. Recognizing that and planning to make the best of both the bad situation and the good situation is positive thinking.
Thinking "I'm going to start this business, I'm going to throw everything I have at it, but I'm going to pay myself a fair salary and save. When I succeed, I will be rich. When I fail, I'll be in a comfortable position to start again. Life is good." is positive.
Thinking "I'm going to start this business and throw everything I have into it, and if it starts to look bad, I'm going to throw even more into it until I'm a hollow shell of a man who can't live with the idea that I could fail" is ignorant. Wishful thinking, but there's a difference between thinking positively and being ignorant of the circumstances.