The mobile fortress is a top-grade rank 5 artifact. That means it has a tolerance limit of 10B.
That is ten times more powerful than what the Sun King can dish out. So while they can't harm the Sun King because of the limit of the world, the Sun King also can't harm them however he wishes.
Only the special bomb made out of the special rock that tears through energy shields like butter can hope to breach the defense of the mobile fortress.
In the past, the Sun King would have thought it too shameless and beneath him to use the special bomb because it meant admitting that he was weak. But after struggling impotently to stop the mobile fortress, he changed his mind and thought that the bomb wasn't so bad.
The Sun King was willing to stoop to any level to open up the invisible shell around the mobile fortress and smash the runt of a man that mocked him into mush. So he decided to leave to grab one of the special bombs.
