That’s not really an architecture though, it’s just basic principles of modularization.

True, which is perhaps why it's unclear what the difference is between a "skinny waist" and hexagonal architecture, "ports and adapters".

hexagonal architecture gives you the direction of the "waist", not its thickness.

You could have a really skinny interface (one method call only):

    class TaxLogic {
        data = iDatabase.loadOneDatabaseRow(id);
        process(data);
    }

    interface IDatabase {
        loadOneDatabaseRow(id);
    }

    class PostgresDb implements IDatabase {
        loadOneDatabaseRow(id) {...}
    }

But this violates ports and adapters because the logic is on the outside and the database is on the inside.