[JadedBlueEyes]

I think what they mean by this is that EdgeDB's query language should not be coupled to Postgres, but EdgeDB itself should use Postgres specific SQL features to maximise performance - so you couldn't drop in MariaDB without changing code in EdgeDB, but in theory you could write another backend that takes the same queries and uses MariaDB or MongoDB or something custom under the hood.

[colinmcd]

Exactly, implementation coupling vs interface coupling.