He Kao originally thought that the portal would be a more suitable place for cultivating the Hidden Moth Temple, but he discovered that what he was cultivating should have been called the Nihility View, which was only the entry-level technique for the Hidden Moth Technique.
If you want to cultivate various techniques of the Hidden Moth Sect, it's mainly in the real world outside the portal. The reason is simple: the state inside the portal hardly matters whether one is stealthy or not, as there is no concept of "movement," nor is there any content to perceive.
The portal is primarily a place for inheritance and comprehension; when facing an impasse in the real world or about to break through, one might attempt to enter the portal for cultivation, but it isn't suitable for most times of the day.
