That's easy to answer, instead of using a relational database or even PostgreSQL's JSON data store, it's using MongoDB. I hate MongoDB but for tests like these where there's absolutely no writes what so ever, Mongo is going to fly. Hence, I read this as comparing MongoDB read performance to PostgreSQL, MySQL or SQL Server perf (or any of the other DB that are listed.)

That's why benchmarks like these have to be scrutinized. I like looking them over but in reality, they're not Apples to Apples.