He hid Jiang Lu's Magic House after the Netherworld became independent, but before Emperor Yin's rise to power. During that time, the entire Netherworld was Jiang Lu's playground. Even after Emperor Yin took over, many secrets of the Netherworld are probably still unknown to him, as the cunning Jiang Lu had destroyed all related documents early on to cover up his many sins. The disappearance of Jiang Lu's Magic House was just one of countless lost records.
Therefore, Jiang Lu guessed that this was exactly why Emperor Yin had no clue about the existence of the Magic House, which allowed that passage to be used to this day.
This was a good thing, but also not so good when you think about it. In fact, Jiang Lu felt a bit sick about it.
If even something like the Tanmo Royal City was unknown to Emperor Yin, then it's likely he was unaware of many bad things Jiang Lu had done.
