The problem with history is that it is written from a person's recollection or understanding of what happened. Most of history exists as a result of interpreting a primary source and constructing a "history" of what was stated.
For Miller, and his play The Crucible, historical accuracy is an issue. Though stating that the play was based upon "sources of varying reliability," Miller admits that there are flaws and errors which exist in any historical recollection or, so-called, factually based documentation. True history can only be written and defined by those who have lived it--not through a person's interpretation of the primary source.
That being said, Miller's admittance that there are issues within the historical understanding of what happened during the Salem Witch Trials, he is telling readers not to regard the play as a historical fact. Miller, in preparation for writing his play, certainly researched what historians stated happened. But, in order to state that his play is "not a history," but relies upon historical events.