They can't. The first wave of dwagons took out the siege because they could disengage from ground units defending the siege units after they had taken out the seige units in a hex. When they got wounded enough, they would be cycled for other dwagons. The "archer hitsies" have been revealed to be massive injuries to the dwagons over the last few pages. And this was done with the dwagons having warlord bonuses.
No warlords means the seige cannot be taken out.