I think it would actually be counterproductive for us to have some sort of very strict rule about what is on topic and what is not; in any case we haven't evolved that rule yet. Nonetheless, I will attempt to explain why I found this question off topic.
As a general principle, I don't think that we should consider the existence of other SX sites (or other sites in general) as relevant to our criteria of on-topic-ness. Given the vastness of the Internet, it is madness to attempt to define ourselves relationally with respect to it. Even within the SX microcosm, that isn't how things should work. For a community to form, there needs to be something internal that makes it cohere; we can't just be the residue that other language-related SX sites don't soak up. (This is the same reason why we ought to accept linguistically informed questions about English, even though there is an English Language SX.)
This site is about theoretical lingusitics. A question about the etymology of one word in one language is not within our scope since it has no way of linking up to a broader theory. Maybe the word was inherited, borrowed, invented, etc., but that fact is a historical accident no bearing on any other aspect of the language or languages in general. Etymology questions can be theoretical, but they have to ask not just for the history of a single word, but rather an explanation of the behavior of a class of words, a clade of languages, a particular sound change, etc.