Hi Rahim, thanks a lot for the reference. I did check them out, both the demo video and a few libraries. You are right that the basic premise is the same. And, yes, they allow for a much more customisable flow. Lots to learn from them!
I learnt just enough web development to be able to build this. So, it will take me a little bit of time to understand code written by a Google team :)
Congrats! Seriously, this is the best way to learn. We're all on the same journey :)
EDIT: Yes, Oppia doesn't appear to update the URL after each step. So if you refresh the page you're back at the start of the lesson. (They didn't have to do it that way, though: http://www.codemag.com/article/1301091)