He lived in 400 BC.
Prior to that history was considered so mixed up with propaganda and myth that it is not considered reliable.
I think they are overstating the case and that a good deal of what they consider myth and propaganda was actually what happened. In fact they have found that to be the case more then once.
Sorry for my lack of ancient history background. I had history along he way, mostly American, but my degree was in accounting and I have been in banking related businesses for over 40 years.