I say that he is one of the two that died.
In Kates episode Jack is on the stand and says that 8 people survived but 2 died soon after. I think this is the story because when they finally come back to the real world that they bring two dead bodys with them. Jin and Maybe Claire(based of the vision that Des had where he saw her get on the helicopter with Aaron).
The reason for the date on the tomb stone saying that he died in the crash, is maybe they had to explain that he died after the crash but they did not necesarilly know what day that he died and the reason he died were from injuries caused by the crash so maybe they just said he was legally dead on the day of the crash.
So 8 people get off the island but two are dead.
Kate, Jack, Sun, Hurley, Sayid, and Aaron.
the two dead people that get off are.
Jin and Claire.