I'm pretty sure he wants to say that you should abstract your database away so that your business code / domain model doesn't depend on it. It becomes a "plugin" to your application and you can easily switch it just by writing another implementation.
And then you have to maintain your own consistency layer in the application because you'll either duplicate data or use relations in Mongo like you would in a relational database.
That's not the problem in my opinion. The problem is advertising any kind of document DB as a general purpose storage. If you have a use case of making different aggregates from data, you'll have a bad time duplicating data or creating your own consistency layer.
I just spent whole day yesterday trying to upgrade RN0.40 -> RN0.42. It's a nightmare because there are conflicts in xcodeproj file and you have to manually merge them.