Luffy said many times that he would be the one to decide when he would die. So no, no one truly killed Luffy yet.
Death is not determined by your heart coming to a stop, it is the death of the brain that determines death. Luffy's brain didn't die in either case.
With Aokiji, he was simply frozen, and once he was thawed correctly do damage was done.
With Kaido, He probably lost consciousness, maybe his heart stopped for a moment from the shock, but his brain was not damaged, otherwise his DF would have left his body to appear somewhere else.