1) skill development through practice and peer review 2) skill development through exposure to "real world" problems rather than the toy problems of school 3) portfolio development for job hunting 4) network building for job hunting
If you're in University, you should be working on open source projects for these reasons and others. I suspect the article author is trying to help spread this meme. Open source contributors really do have a major advantage over their peers, but many students do not realise this.
Others don't know how to start, but that's a different problem.
Besides, open source is fun, and you probably have way more time and resources during University than you will later in life.