Even the article you just linked me to makes note that dead Christian is not the real Christian, but MIB posing as Christian. We never see Christian's ghost until the flash-sideways, which is confirmed to be purgatory. To say the island has to be purgatory because we saw something that looked like Christian is the same as saying the "Why is Wolfe barking?" scene from Terminator 2 is purgatory because we see John's stepmother when we know she is dead.