The issue it not only the documents but their content.

        The writings of Josephus appear at a known time with a known creator whose purposes are often explicitly stated. Thus we have a traceable document with a known content.

        The Septuagint appears to have been created around the mid 2nd c. BC but the dating of the stories is largely unknown. What is known such as Daniel is consistent with the mid 2nd c. BC.

        The version of this in Hebrew is known only by reference until the 4th c. AD and whose purported existence prior to the Septuagint is based solely upon a forgery (Letter of Aristeas) which is also dated to the 2nd c. BC. What is today called the Hebrew bible does not appear in history until the mid 11th c. referred to as the Masoretic text.

        The Masoretic is an abbreviated form of the Dead Sea version. The Dead Sea version is in turn an abbreviated version of the Septuagint.

        So whether or not one likes Josephus he is the only known historical source and BTW he presents a different version of the "old testament" than is found in the Septuagint, the DSS and the Masoretic. Josephus professes no knowledge of a source collection for the Septuagint but rather refers to temple records. Although he presents Aristeas letter as evidence of a perfect translation he gives no indication of knowing of a specific collection that was translated.

        It is like he knows there is a problem and bluffs his way through it. He indicates knowing of a possible source for the material (generic temple records) but knows of no particular collection of records which matches the Septuagint collection. And although he tells different versions of the Septuagint books in Antiquities he does not include "temple record" materials
not found in the Septuagint. That is, he tells a different version of Exodus with Moses as a military leader but he does not tell of a book not found in the Septuagint.

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For the record I post material like this in hopes of people who have found material I have not which is contradictory will tell me about it. Different opinions about the material are a dime a dozen.