I have not found any clearly stated policy.
We cannot comment on the handling of questions that have been asked on Statistics.SE. You'll have to contact the folks on Stats.SE about that. The community on Stats.SE sets the policy on what questions are on-topic on Stats.SE. The community here on Math.SE does not control that.
This site is only responsible for the scope of Math.SE, and thus the Math.SE community can only answer which questions are on-topic on Math.SE. The Math.SE community does not determine policy about which questions are on-topic on Stats.SE. Not all questions have a home somewhere on SE; some questions are neither on-topic on Math.SE nor on-topic on Stats.SE, and some questions are on-topic on both.
Based on our help center, some questions on statistics might be on-topic here, but might be better addressed on Stats.SE.
I have not found any authoritative statement about which questions related to statistics are on-topic here. You might read the following resources for discussion and possibly some guidance:
Ultimately, in case of ambiguity, I suggest you pick one site where you think your question is best-suited, and ask it there. You might consider whether your question is best answered by mathematicians or statisticians.