Sun and Jin shocked to see Michael

Here we see Sun and Jin's reaction to seeing Michael aboard the freighter. I think to say they were shocked would be an understatement. Michael tells then them that he and Walt followed the bearing Ben gave them and found an island with people. He says they sold the boat and hopped a cargo ferry back to the United States. After Sun asks him if he's now working for Ben, he tells them that he's trying to make up for what he did (killing Ana Lucia and Libby).

This explanation of how he and Walt got back to the states is a lacking details. Is he telling the whole truth and will we ever see how these events played out?