This is a great question to ask, because in his preface to this excellent work, Dahl himself says that this is "not an autobiography." However, let us be aware of how he defines autobiography:
An autobiography is a book a person writes about his own life and it is usually full of all sorts of boring details.
Dahl claims that this novel is not an autobiography because he would never "write a history" of himself. However, at the same time he says he will communicate a series of incidents from his life that have remained firmly in his mind. He final words in the preface are that "All are true." The definition of an autobiography is a text that tells the life of a person that is written by that person. So, in spite of Dahl's protestations otherwise, this is an autobiography, and an excellent example of one. A biography is a text based on a person's life that is not told by that person.