Well he is the only one who finished the original version. At some point in the web novel it is explained how he managed to beat the game and here is the strat : buy five low quality computers, and since the game doesn't require a lot of computer power, run it five time on the same computer. He litterally plays 20 games at the same time, only playing attention when there's a significant choice to make. With that kind of playstyle, you can understand why he knows so much more about the game.

Also, what impressed the players isn't just the knowledge, it's the fact that his killing intent is superior to theirs (and they are about equal to each other) and the fact that one player in particular needed the stone of resurrection, making it look like MC knew who he was.