Nothing wrong with Rails, especially if the team knows it well. Time to develop is the real cost in software most of the time.
If Github wanted to integrate a lot of real-time features, then Elixir + Phoenix can't be beat. Depending on what they replace, a 10x in performance and a fraction of the servers needed is a nice win.
I've seen performance boosts closer to 20x when I've helped companies rewrite their Rails products in Elixir. I've also seen a reduction in server costs. In all fairness though, simply rewriting the Rails app in Rails, with the benefit of hindsight, probably would have resulted in a performance gain too.
If Github wanted to integrate a lot of real-time features, then Elixir + Phoenix can't be beat. Depending on what they replace, a 10x in performance and a fraction of the servers needed is a nice win.