The question linked in OP is borderline but not clearly off-topic
The querent did in fact first ask on math.se -- notably with different text with what seems to be an attempt to use more mathematical language. At that time I linked a relevant existing question as a comment, but the question was neither closed as duplicate nor directly answered. Apparently the querent either did not see my comment or did not find the information sufficient, as they later asked here on rpg.se, bringing us to the current situation.
I neither voted to close nor to (re-)open the question here. I think there are a couple of on-topic-for-rpg.se questions trapped inside, struggling to get out; this is why and how I answered the question on this Stack using RPG context, but there is a continuing breakdown in communication.
math.se doesn't want context-less questions either
This is point #1 on math.se's "how to ask a good question" FAQ. Actual question closure and de-duplication on math.se is spotty at best, but to be fair math.se has a larger flow of questions over a wide variety of topics, so the task is genuinely more difficult. Arguably, then, the criticism should be "this question does not provide enough context for any Stack" rather than "this question is off-topic and should be asked on math.se or stats.se instead".
The mere act of posting on rpg.se signals a little context
While insufficient on its own to make for good context, choosing to post here rather than on math.se or stats.se is not nothing. For example, if you post a statistics question here, you are likely to get an AnyDice answer, which is a well-known tool within the RPG space and is usually close to ideal for a RPG context. Meanwhile, on math.se most people have never heard of AnyDice; you are more likely to get a probability generating function (PGF)-based answer, which while elegant from a certain mathematical point of view, is probably not as useful for a RPG context. (Though if any PGF enjoyers would like to make an argument for them in a RPG context, I'd love to read it.)
For contrast, here is a recent question where the specific mathematical frame really is the focus rather than the RPG context.
For the question linked in OP, I chose to list several physical realizations of a d7 due to the RPG context. I do wish the querent was more clear as to whether this was what they were looking for.
Expertise tends to favor rpg.se
I've been monitoring the dice tag on rpg.se, math.se, and stats.se near-daily for the past two years. If you post a dice question, there's a chance it will go unanswered on math.se, whereas anydice and statistics on rpg.se have a perfect answer rate, with dice just one short (and not due to lack of mathematical expertise). If the problem is easy, many people on rpg.se can answer it; if the problem is hard, you're better off looking for specialization than general math expertise, and I think that's going to be easier to find on rpg.se than anywhere else. RPGs are a narrower topic than math or even just statistics, so the intersection of the two is ceteris paribus going to be a larger fraction of RPGs.
I would personally prefer to answer the same question here than on math.se. The community and its moderation are stronger here, and contributions get more attention. Though I recognize that these rely in part on maintaining the quality and focus of questions.