The people of the Cangyu Sect can only enter when the Absolute Formation Gate opens every three years; only then won't they be killed by the power of the Absolute Array.
The time the Absolute Formation Gate opens is two months.
That means only during these two months can the Cangyu Sect be as free as the Demons.
Sikong Jing, of course, also asked, what happens to the Demons if they leave the Divine Demon Island Sea Domain and enter other parts of the Desert Sea Absolute Array?
The War Tiger's answer was: they would also be killed by the Absolute Array. However, if they reach the Holy Martial Stage, they could resist the array, but the Cangyu Sect doesn't allow them to achieve that, so the Demons cannot leave this sea domain of the Divine Demon Island.
In plain terms, it's because once the Demons reach the Holy Martial Stage, they can move freely throughout the Desert Sea Absolute Array.
