The author has not charged money for his app yet so while we know some users like the application, it doesn't tell if they are willing to pay for it.
I agree that it depends on the customer whether you should charge right off the bat or not, but the article does indicate its a paid site now:
"Then after two months of free – I turned the site paid."
"Subsequently Pluggio retention rates of paid users are thorugh the roof – with the average paid user sticking around for six to nine months."