They use a custom mysql thingy. An old version of the MySqlConnector; where it was still reliant on the InteropRuntimeServices thingy.
So like mysql-async 2.0.2 but I only saw sync calls (was only skimming that). It is performing under stress very likely less well than my MySQL, which they could use as is; if they modify it, they would have to publish it (AGPL 
), so they very likely can’t.
But they got instantaneous response times for that instead of having to wait a server tick for response.