Something like this could be considered historical evidence, but not necessarily. For example, I could write a fictional tale about a friend of mine and someone could discover it a thousand years from now. But this would not make it evidence to any real truth, would it? This is one of the problems with history and historical documents. Many of them are very doctored up with tales. Kings would often have embarrassing made to look less embarrassing or to look quite the opposite in any historical writings that took place in their life time, etc.

There is also the problem of validating if indeed the writer of a document was really a person that actually knew the individual they wrote about. It was often very popular in ancient times to write stories about well known people (fictitious or otherwise) and to even publish a work in a famous person's name. An example of this is the book of Enoch. The book claims to have been written by Enoch, the famous individual from the book of Genesis, but it is many hundreds of years newer than Enoch would have been.

This sort of thing causes problems for Bible scholars as well. For example, the various books of the Bible are attributed to specific authors. The Gospels, for example, are said to have been written by Matthew, Mark, Luke and John. However, it is a matter of debate (even among Christian scholars) as to who actually wrote them and even WHEN they were written. So was Matthew written by the disciple of Jesus who knew him or by someone else? And even if many consider it to be written by Matthew, how would we know if it wasn't written by someone writing in his name?

The same problems are there for the Qur'an as well. Who wrote what? And how do we know for certain that such-and-such is really the author?

Contemporary writings do factor into history, for sure. However, they can be problematic and, as such, are not completely reliable. Even if they contain truth, this does not mean that everything they say is truth. For example, people derive historical concepts from books like the Iliad, but don't believe in the monsters therein.