The Battle For The Liberation Of The Plane.

Aeternus might have a flaming skull but it isn't enough to scare them or take him seriously. Besides, he is just one demon, he might be able to go against the full array of the god's resources in the mortal plane and win, but it should take him time. It is a win for them as long as he is stalled. 

They plan to delay him by dragging out the battle. It doesn't matter if they win. They know that but it is very unpleasant for them to hear their impending defeat from the mouth of their enemy. It is even worse that he said it in front of all these mortals. What if one of them survived this battle and tells the world? 

It will be like a mortal witnessed their shame and lived to tell it. No god wants to be disrespected in front of mortals. They used to kill mortals for witnessing their shame. So now they have to say something about the disrespect or make sure that no mortal will survive this battle.