CoreData is an Object Graph Manager which can work with different store types, with SQLite being the most popular one as it is persistent. It provides graphs of 'NSManagedObject'.
Is Realm also an Object Graph Manager as it provides graphs of 'Object', with its own implementation of a database management store? If Object Graph Manager and DBMS are two different things why Realm claims to be a DBMS instead of claiming to be a both?
Also in general if your own custom designed database interface(a custom solution as an option to CoreData and Realm that you might implement) provides objects linking to underlying records in the custom store implementation(Some object Similar to NSManagedObject for CoreData and Object for Realm) isnt it a Object Graph Manager with a underlying store or is it a DBMS?