You can implement a GraphQL service in anyway you desire. There is no inherent relation to the storage solution you use.

GraphQL isn't anywhere close to being similar to SQL, so I find the desire for an analogy very confusing.

To me, these are grammars for interacting with an API, not an API.

To me, it is like calling a set of search parameters in a URL an API or describing some random function call as an API. The interface is described by the language. The language isn't the interface.