That does not mean that the things they write are always or even usually true. It means that the historians think they are true.
This is an important distinction because, for the last few decades, much academic writing about ancient and medieval historians has been based on the assumption that they usually lie. The assumption is understandable, because so often two histories contradict each other, and our natural human reaction is to say that at least one of them is lying. But that reaction is usually wrong.
Curiously, we do not usually react that way to current works. We know, for example, that there are people who deny that the moon landing ever happened. They write long volumes of half-digested incorrect history to prove their pet hypotheses about what actually happened, citing mounds of misunderstood evidence. But they are not lying. They are simply fools. We understand that instinctively.
What Dr. Boli is suggesting is that we ought to treat historians of former ages the same way. A few lie, mostly the ones who were themselves participants in the events they describe. But those are very few; and furthermore, if, like Caesar, they are competent writers, they usually get away with it. Many others give us wrong information, but that is because they were misinformed.
From a practical point of view, what this principle means is that, when we detect a false statement in an ancient historian, our work doesn’t end with calling him a liar. Deliberate deception is so rare that we should dismiss it as exceedingly unlikely until we have exhausted every other possibility. Instead, we have to ask why our historian believed that statement was true, and find where he might have got his false information. In doing so, we may find some pathways through history we had missed. We may discover that Nero the tyrant was also Nero the populist, for example, or that Heliogabalus the hedonist was also Heliogabalus the religious devotee and feminist.
The delightful thing about having one’s own Magazine is that one can make these statements with no editor to say that they won’t fly, and no peer reviewer to demand that one show one’s work to the fifth decimal place. But there is a comment section, which might be described as ex post facto peer review. Commenters are welcome, as always, to disagree as vigorously as they like. They may even call Dr. Boli a liar, but he will deny it. The most he will admit to is being misinformed.