The defeated, large human army had to come from somewhere. That somewhere is now much more lightly defended, with the destroyed army out of the picture. On the orther hand, the demon army here was but a fraction of the size of the human army, so it wasn't drawing that much strength away from the demon lands.
Ike could leave the other demon generals to deal with the red dragon knights and push forward to capture the places this destroyed army was supposed to defend. Unless the humans are simply 10 times more numerous than the demons, and this destroyed army doesn't even matter pretty much at all.