Okay, let me rephrase. You said they have no thing except a world history text book as evidence of our existence, and which one's right. I say they are both false. "One being says the book is most likely fictive due to lack of any "real" evidence", this is false because the book is a real evidence. "another says they can't make any assumptions either way because the book is the only evidence" This is also false because they can make assumption because the book is enough of an evidence. If we don't exist, no one would have wrote that book, so whether that book is fictive doesn't matter, its existence itself proves our existence.
"If they found both a history book and a religious document, they would have to come to the same conclusion regarding the validity of both (assuming they are unbiased, of course)."
That is not true, the contents of history books are usually supported by many other historical text stating roughly the same thing. Everyone have their own view on the world events, so exact match is very hard to find, but if you see many texts stating roughly the same thing, you can infer with much confidence that it did happen.
On the other hand, when people see so many different contents in the different religious texts, people tend to think the facts on those texts are unreliable.