Byakuya took more of a beating than Kira did though. And even if the 4th Squad wasn't sent out until after the Quincys left, we've seen instances where ex-4th Squad members who've moved to different divisions can still use healing techniques. When Matsumoto got her torso ripped out by Ayon we found out that Kira was formerly a 4th Squad member and he was able to stabilize her. Who's to say that among the soldiers who were around there couldn't have been someone capable of treating him?
Anyways, like I said, the point is until we get word that he's actually dead or actually alive we shouldn't just flat out assume that he's dead. Byakuya's zanpaktou even broke apart which is usually a bad sign but he still survived. Kubo doesn't like killing off his characters off. If Kira died then who would fill the vacant Lieutenant spot? There's no one who could replace him.