From the FAQ:
You may ask questions about any area of Christianity. Your own beliefs do not not preclude you from asking questions, but all questions must be directly related to Christianity.
Taking the example that you're referring to, that question is about a specific passage in the Old Testament that does not have any relation to Christianity whatsoever. Not even a possible Messianic twist.
Not all Old Testament questions are off-topic. For instance, the Ten Commandments are used as a basis for some of Jesus' teachings in the New Testament. Laws of Leviticus and Deuteronomy may or may not still apply to us today. The first few chapters of Genesis are fodder for on-topic questions as they deal with Creation and very fundamental aspects of humans. The Flood is on-topic as it is a fairly significant part of Christian apologetics, particularly those that assert the infallible/inerrant nature of the Bible.
Off-topic Old Testament questions would include those such as how to apply a specific law (such as not cooking a young goat in its mother's milk), or reasons that God ordered the Israelites to do a particular action, or what a particular symbol represents in Jewish culture, or other similar questions. Basically, if its interpretation can be sourced from and contained within the Old Testament or other Jewish works, it's off-topic.
An edge case example: asking about the law of keeping the Sabbath holy by doing no work on that day. A question that was simply seeking a clarification on just what exactly constitutes "doing work" would be off-topic for this site, but a question asking how Jesus healing a cripple's hand constitutes work might be on-topic. Asking why Jesus ignored that prohibition on doing work on the Sabbath would definitely be on-topic.
A person asking a question on a Christian site may implicitly expect a Christian audience to answer, but a Christian audience is not necessarily the right audience to ask.
And yes, if we migrate a question from here to Islam.SE (like, say, whether Muhammad believed Jesus was the Son of God) and you still wanted to answer it, then yes, you would have to answer it over there. Also, SSumner didn't previously have an account on Mi Yodeya, and it is unreasonable to expect that everyone will know the best place to ask questions. That's why we have the migrate feature.