Databases aren't simply persistence layers, they also encompass querying which implicitly couples your application to them. The simplest query model is the key

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.