History of Science and Mathematics moderator here. We've talked it over and we've sent the question back to Math Overflow (I closed it as off-topic on HSM, so that should bounce it back to MO automatically).
We've had a lot of conversations with other STEM sites about their policies on historical questions and what any migration process would look like. I think the big takeaway for me is that these questions really have to be dealt with on a case-by-case basis. What that means in practice is that meta discussions for each question can be quite valuable. There's often a fine line between questions that should be send to HSM and questions that should not - it's hard to capture the nuance of that in a few comments.
This case is a great example of that in action. I agree with this meta consensus - the required degree of technical expertise means the question will do better on Math Overflow.