"EdgeDB is an open-source object-relational database built on top of PostgreSQL. The goal of EdgeDB is to empower its users to build safe and efficient software with less effort."
Yeah, that's true - Postgres is the engine. On top of that we have a lot of infra though: our IO server (amongst other things it acts as a server-side connection pool), compilers to compile our query language to SQL behind the scenes and to enforce our type system, a migrations engine, etc.
It's not that uncommon to stack DB engines on top of each other, we're just taking this concept a bit further.