Databases aren't simply persistence layers, they also encompass querying which implicitly couples your application to them. The simplest query model is the key value store, which itself is a form of coupling, but it's quite limited in the operations it can support with high performance.

GraphQL is similarly not a transport layer. The transport layer for GraphQL is HTTP. GraphQL is a query language. To use it with a particular database you need to transpile GraphQL to the query language supported by your database. Usually this is done in an unprincipled manner with ad-hoc code. In other cases it's done more globally and formally, as I assume Hasura is trying to do.