In the Oxford articles, they keep stressing that such-and-such a document is not strictly historical but serves an ideological purpose as well.
Why would they expect the ancients to observe modern rules of historical writing, as if people of yore shared our irrational obsession with objectivity? They were telling the story; that’s all. They were reporting and interpreting in the same narrative, just as you and I do all the time. Even now, people have to break that habit if they want to write history or journalism by today’s standards.
The ancients didn’t share those regulations. They weren’t trying to divorce what they thought from what they’d seen. They were writing naturally.