Interesting that they decided not to do a responsive design. Instead, it serves up different HTML if you send a phone user-agent. I've been trying to decide if this is a good idea for my own project.
For us, it made sense. Our desktop site is pretty large. Check the graph in the blog post- the mobile pages are about 1/10th the size of the desktop CSS and JavaScript. By avoiding a responsive design, we can make things load disproportionately faster for our use case. On top of that, it's isolated so we're less likely to break views by adding features later. Pretty fun.
I'd love to know the technical approach on this. That is, is the different HTML/CSS/JS view called by the Rails controller, based on the request's user agent? Though responsive designs are often a good call, I completely agree with your decision to do custom views here. I'd love to know where in the process the filtering happens, though, such that you retain the URLs (no "m.github.com") and yet have the customized view. Any backstory on the technical aspects that you can share?