Your question indicates your confusion regarding the two separate and different terms of autobiography and biography. Let us be very clear about the differences between these two words: an autobiography is the life of a person written by that same person, whereas a biography is the life of a person written by somebody else and not by the character that is the subject of this work. Therefore your question is an impossibility as a work of literature is not able to simultaneously be an autobiography and a biography.
However, it is important to realise that Dahl writes this not as a traditional autobiography which systematically goes through his entire life. In his preface, he makes his intention clear. He does not wish to bore his readers and thus selects a series of events that he talks about that are not necessarily thematically united. However, this does not detract from the fact that this work can only be considered as an autobiography. It can never be considered as a biography, for the reasons outlined above.