Should historical data be considered ambigious if not proven as fact?
Any time you have a single source for information (generalizations given by a conquering foe), there is no other supporting evidence, and there are no counter-examples, then yes, it should be accompanied with disclaimers since the data is actually quite weak.

It would be like saying "John Smith reported the sky turned red just before the earthquake." This is far better than saying "The sky turned red just before the earthquake." The latter implies that there are multiple independent sources for the information where, indeed, there might only be one.