It's built on Postgres, but it isn't a _thin_ wrapper. We lean hard into Postgres query machinery and type system in order to pull off EdgeQL and graph-relational efficiently.
There are countless permutations of the choices that database designers face, so it's a shame there aren't mature products for more of them. I hope this particular permutation turns out to be a good one for lots of people :)
And you - or hypothetically the end user - could change the backend, e.g. to Cockroach for better horizontal scalability, while trusting that EdgeDB will only rely on Postgres's public API at least in meeting its own public API/contract?
[0] It's hard to make that analogy with Postgres b/c it only has one storage engine, but of course the separation still exists.