That's what i thought at first, but thats a huge weakness and i dont think it would be worth the trouble to make him jump if he is going to open his mouth to gobble up prey anyways. Aiming for the belly is difficult, since the demon jumps at a diving angle. If the author did make the mouth a weakness, it would be too easy to kill. Either that or he was trying to make the monster jump in the direction of iel.