The source is some dude who wrote a book called "The Coming Collapse of China" - a prediction he's repeatedly made since 2001. This article by the deputy dean of the Institute of International Studies at Shanghai's Fudan University explains that Chang is not as he appears:

Chang has made predictions repeatedly and than failed time and again. That is because he has either disguised his true intentions or made the analysis based on a disordered mentality.

I'd bet my right hand Chang is making these alarmist comments as a means to increase his book sales. It'll probably work too. This subreddit is free advertising!