This article is just useless fluff. First, how many people care enough to get 100%? Do you get anything for getting 100%? If not, then the number of people who will probably strive for that and are willing to pay for everything is probably low. Second, it was said in the article, you don't need to buy anything in order to win, so the headline on HN is misleading at best.

I personally see nothing wrong with this approach. Development time is not free and when offering the game for free there has to be a way for the developer(s) to make money. As long as they allow users to beat the game without having to make a single purchase then it's all good in my eyes.