I do not know what it says in the introduction of that book, however I would imagine that his explanation would go something like this.
The story of "Lo Bashamyim hi", exists in a very specific context. That context is a Beit Din arguing with one of the members of that Beit Din. That is, we have a situation, where a beit din is gathered, and they are making a ruling. In those contexts, the Shechinah is said to have descended upon the group. The Majority of that Beit Din are then given the authority to make a ruling, and Bat Kols are not a valid form of testimony to the beit Din.
However, the concept of "Lo Bashamayim hi", does not necessarily apply in a situation where there is no beit din, or there is no sanctioned method of testimony.
There are numerous examples in the Talmud, where a halacha is learned or passed down by eliyahu to a single individual, or through other form of knowledge gained from "the heavens". Never in any of those situations, do we argue "lo Bashamayim hi". The only time that argument is really made, or has any valdity is in a situation of a beit din, or a gathering and a strict vote, which is gathered from numerous talmid chachamim "sitting together" to discuss and come to a conclusion.