The mind and the body are two separate entities. Therefore, if an oracle is a suitable vessel for Dorje Shugden to take trance in, then the enlightened mind of Dorje Shugden can enter the physical body of the oracle and use it as a medium to communicate with ordinary beings which would otherwise not be possible. Therefore, when the oracle takes trance of Dorje Shugden, it is actually Dorje Shugden, the enlightened mind of Manjushri speaking, not the oracle himself.
Therefore, it is not surprising that he could teach and answer question spontaneously and quote from text effortlessly. In fact, once the trance is over and the oracle recovers from being taken trance in, he would not even be able to know or remember what took place during the trance if in fact a real trance actually took place.