The mongodb repo was indeed a direct transformation of the relation model to mongo. The idea was to try out the (then newly released) transactions in Mongo and see how it behaved compared against a relational database. And of course to see if people had interest in contributing and building it further out.
If you look deeper into the code, you’ll see the basics are there but many methods are not yet implemented. As @ylyly states, it would need a revision of the way the data is stored to store it in a format more native to mongodb. So right now it is more in an experimental state than anything else.
If you want to learn more, there was a talk on JAX London about it last year: https://www.youtube.com/watch?v=vzgU1lZ1h3U