For the younger members here I think it's hard to imagine just how freaking cool SourceForge was when it first came out. The idea of free, featureful project hosting was amazing. Back in the 90s, even if you had free hosting, there wasn't the selection of easy-to-install FOSS web software that we take for granted today. It's sort of like how cool Slashdot was in a world of Usenet and BBSes. Amazon, Ebay, Yahoo. It's easy to forget how impressive Web 1.0 was for its time.
GitHub by contast came out of the Web 2.0 era where we had solid CSS, solid JS/AJAX, solid web frameworks, and a lot of hard-earned knowledge of UX design, and perhaps more importantly the browser technology to support it. I think the article makes a good distinction between code hosting and project hosting, and no doubt that makes GitHub's job easier, but it's not the whole story. GitHub is the result of a holistic vision that goes from the metal up to the UI and design bits. In the early days of the web putting together a team to do something like that was well-nigh impossible. With the ubiquity of the web today, the new generation of web folks can much more easily pick up both the technical and design skills to create the kind of amazing UX experience that sets GitHub apart from SourceForge.