The guy passes through walls and falls through the floor and this is what breaks your suspension of disbelief? The game world already had a lot of arbitrary game breaking bugs - some inconsequential and some plot-convenient; another one being a coupling of two unrelated game objects isn't going...
Come to think of it: why did the demon only tied up the shrine maiden and not kill her? I thought the demon didn't care for human lives. Is the dragon so omniscient that it could swoop down and attack the demon as soon as he had the intent?
His actual cheat ability is actually manifesting itself midfight rather than just during training. This is something his masters wouldn't be able to teach to him from their own experience. I suppose the real catalyst to this is having learned self-healing abilities with the mana to support it.
The culture too: things like greetings, respect for seniority, intimacy and boundaries, etc. Some are subtle and nuanced, but even if you can't quite put a finger in it, it's gonna feel different when you see it like say, the differences between a manga and a manhwa.
But then if you think about...