The source code is available for inspection as is the biographies of the people involved.
They rely on the libraries that are part of Postgres itself to ensure they are parsing the SQL etc "correctly" (where "correct" means "the same as Postgres itself).
Bigger organizations do not necessarily mean higher quality.
What bigger organization is testing PostgreSQL itself?
What are the relative quality measurements of Postgres vs MariaDB vs Oracle vs SQL Server?
MariaDB has over 200 employees.
https://mariadb.com/about-us/careers/
Oracle's department that handles DBs probably has at least a few hundred.
3 people would be an ultra lean QA department for a product like this.
I'd have a hard time convincing my boss to go with PGDog over a more stable and tested solution.
This doesn't mean it's bad, just not ready yet