So JOIN in your application. Or use a different DB. MongoDB isn't correct for every use case and they would never claim it is.
I can understand calling something that holds your data and makes it so you don't have to do JOINs a database.
I can understand calling something that holds your data and does JOINs for you a database.
I'm having trouble calling something that holds my data and makes it so I have to "JOIN in my application" anything but a file system.