I thought MongoDb:s main selling point was the simple scale-out model? At least that's what stuck with me. I'm well aware about the non-robustness properties of Mongo, but to me it seems like calling this a clone without the scale-out capabilities would be missing the point.
It's a proof of concept. The author's essentially saying, "here, it can be done, this is one way of doing it." It would need more bits and pieces and more tuning to be useful in a production case, but this shows that it at least can work.
I think you missed my point. The USP of MongoDb in my view is the simplicity and its scalability model. At least these are the things which are interesting to me and what is hard to implement. They have cloned a subset of the functionality - the subset which is easy - and left the hard parts. I don't agree that this proves anything.