"Every serious editor has solved displaying text and styling since ages ago..."

Sure, but why reinvent the wheel? In this case, Github is leveraging a technology specifically optimized for text and styling. They don't have to maintain it, and they get a lot of cutting-edge optimizations for free. It also gives them the freedom to do some crazy things that would otherwise be very difficult — see Light Table.

  "...and those two points aren't even that important in programming languages."

I disagree. Efficient text display and presentation in a text editor are very important, at least for me. Not that scripting is any less important, but it looks like Github has that front covered as well. (Then again, I've never been a heavy vim or emacs user. I guess I'm what you might call a "filthy casual".)

  "One thing that is incredibly important for text editors is speed: something web engines are notoriously bad at."

I was under the impression that web engines are really good at text when there's not a lot of other crazy nonsense going on. Guess we'll see!

  "So why is every text editor I have ever seen embedded in a website an abomination?"

My guess is going (partially) native and using Node.js will make the experience a whole lot better.