from what I have read he must have fallen inside the dungeon without knowing and he thought he got transmigrated to a new world and was not inside the game. yes I know it is very confusing with almost no information to go around.
ok guys I am bringing news for you guys this story that look like a other story it is still part of the game. he is currently inside a dungeon and the last chapter for this dungeon is 95 so it is 25 chapter long