And indeed as many have guessed, the entirity of the fleet TIE force has turned up, the sky almost going black under their numbers.
The bombers bomb the bridgre first, the thermal detonators and other warheads blowing apart the mighty structure even as thousands or more are on it. The water, full of deadly predators, is of little sanctuary to those who jump off, but even if it was, this was not the end of the matter. The bombers are ordered to exepnd all of their warheads, bombing the river and the ground either side of the bridge over and over and over.
The fighters are strafing. They are told to strafe everything that moves and everything that does not. The Osokans have no chance of outrunning the fighters, and if they miss they come around for another pass and do it again. They have all the time in the world.
If the Osokans stay up, the fighters hit them again. If they go down, the fighters go and strafe the bodies anyway, just to make sure.
Vader's tactic was based on the idea that a panicking enemy fleeing in all direxctions can slip throuugh your lines and be lost. But leave your enemies an escape route that you know they will take, with a point where you know you can slaughter them all- then victory will be truly total.