The key question for me would be whether this or any other Mongo alternatives are compatible with mongodb's indexing capabilities. They are complex, powerful, and can be somewhat finicky to understand and deploy.
As a longtime Mongo user who was disappointed to see their licensing restrictions, I would probably go back to the release before the license change and maintain a fork rather than try to plug in postgres under the hood.
We actually considered it at Percona with few other companies in the space and there were not enough interest, unlike in Elastic case.
Keeping reasonably up to date in Tech with 30B+ company by reimplementing key features from scratch is daunting task.
MangoDB approach focuses on PostgreSQL (and PostgreSQL Compatible) databases future innovation which makes long term progress much more feasible.
With DocumentDB and many other partially MongoDB compatible software getting traction I think we will see subset of MongoDB functionality to emerge as de-facto standard in this space
As a longtime Mongo user who was disappointed to see their licensing restrictions, I would probably go back to the release before the license change and maintain a fork rather than try to plug in postgres under the hood.