How are they not highly advanced? They are the foremost faction in space exploration and have taken technology from all over Sirius. They have some very nice firepower while at the same time being a mobile home for those onboard.
Apparantly it's only one of those said Juggernaughts that are keeping the Corsairs away from one of their Freeport's.
Take the ones away from other Freeports for instance that are not as much at risk and you have a small fleet.
The corvo is still a ship, even if it is sold to everyone.