The article isn't saying you shouldn't be passionate about your work, rather that you shouldn't restrict yourself to selling in markets where you have a personal interest.

I have mixed feelings about this. I started a website about senior care giving because I thought there was a market need and advertising rates were/are incredibly high. However, writing about senior care as a 27 year-old male is mind numbing. The site failed because I had no motivation to create content for it. So, the lack of passion can definitely result in failure.

It seems to me that the key to success (as noted by the article) is finding where you can add the most value. I'd say that:

Value Added = Function(Knowledge, Work, Market Need)

If you work on something you are passionate you have a head start on the first two. But, this doesn't matter if the market isn't there. The trick is finding market needs where you are uniquely suited to add value. In a soundbite, you shouldn't follow your passion, but you shouldn't ignore it either.