Caveats: I made a couple of comments in the hope that this meta-question could be improved before answering, but you've declined, so I will go with it as it stands. Also, I didn't close the original question, so I am putting words in @Sklivvz's mouth, which is dangerous.
There are several questions in this meta-question.
You ask in the title "No expertise exists in the humanities, only opinions?" This is a strawman. No-one has alleged this or acted as if it is true. The answer appears to be "False".
There is a related question, suggested by your claim "I have a background in religious studies", which is "Doesn't my expertise count for something here?" The answer is pretty much a categorical no. The answerer's claims to be an expert are irrelevant for two separate reasons:
It might not be true. Trolls lie. People exaggerate for good reasons. We have no way of checking the qualifications and background of a random Internet denizen. So, we have to assume they have no expertise.
Experts get things wrong. Sure, they are much more likely to be accurate than a non-expert, but they still make mistakes and have biases. We don't just need to hear from an expert - we need to hear how an expert reached their conclusion. Ideally, peer-reviewed by other experts, to greatly reduce (alas, not eliminate) the chance of mistakes.
This is where your expertise is incredibly helpful. It should allow you to quote high-quality references to support your argument, and to explain what those references are saying in clearer terms, because you know the area.
However, simply saying "I am an expert" counts for nothing here. If I am already editing an answer, I am likely to drop such claims as irrelevant.
You argue that the question is not opinion-based. I suspect (I haven't checked!) that @Sklivvz was influenced by the line in your answer:
One can debate endlessly over which of these stories is more plausible. There can be no resolution to that.
Your answer demonstrates that there is no definitive answer to this question - just people bickering over it from their own religious perspectives. This is an anathema to Skeptics.SE, where we want to provide definitive answers to questions, by applying scientific skepticism.
Is there no such thing as a fact for a broad question about the reliability of textual sources?
First, I want to emphasize, there are Stack Exchange sites where they use different mechanisms to increase human knowledge. (Scientific skepticism is not the only choice. I often give Courts of Law and Mathematics as two alternatives that are widely respected, but have different goals and different rules for finding the truth.)
For this theological question, I suspect Islam Stack Exchange is the best bet.
If it was about the Christian Bible, Biblical Hermeneutics Stack Exchange would be an even better place - a place where people can argue over the most likely meaning of a theological text, without the obligation to provide a single definitive answer based on evidence.
History Stack Exchange is another example.
Undoubtedly, some questions in the Humanities challenge us here, as difficult to answer given the self-imposed constraints. There are claims made by my high school English teacher about Tess of the d'Urbevilles which I think are nonsense, but Skeptics.SE would be a terrible place to discuss them.
Looking at your answer to the closed question, in particular, there are very few "facts" provided, and most of those are unreferenced.
Nonetheless, there is not a blanket ban on questions in the Humanities, as the history, literature, religion and movies tags, for example, reveal.