They do have a very long detailed post about that, and it covers the poor error messaging too.
April was a single player Alpha. It was limited to the first two acts and had a small player base. Being offline it did not test the Global Database functions.
Closed “Early access” Beta was one weekend in August.
Open Beta was a second weekend in August.
Those were both Multiplayer and did test the Global Database, but given the limited scope of just two acts, the short duration, and that nothing from Beta is kept, it did not have the player participation Live does.
So yeah the test phases were small.
EVEN so, there were issues identified there that were not fixed for Release.