We should be inclusive. Anything is fair game as long as you're doing it in Emacs. It isn't up to the asker to know whether what they're trying to do in Emacs is going to be solved through a core Emacs feature, or with an extra Emacs package, or by invoking some operating system functionality via Emacs, or by doing something outside Emacs altogether.
This is the same policy that sites such as Ask Ubuntu, Ask Different and Unix & Linux. Questions about third-party applications are welcome there. It can happen that the answer to a Firefox question on Ask Ubuntu will be equally applicable on Windows, but it can also happen that the answer will be specific to the Ubuntu setup: as long as the asker is running Ubuntu, the question is on-topic.
If Emacs is not involved at all, then the question is off-topic. For example, a question about using Git from the command line would not be on-topic merely because the asker wants to edit the files in Emacs. But a question about using Git via Magit is on-topic, whether or not the question is fundamentally about the underlying Git setup.
It may happen occasionally that we detect that a question requires non-Emacs expertise, that Emacs is only incidental, and that the question is best shown to other experts (say, git experts). In that case, especially if nobody has been able to answer the question for a while, a good course of action is to suggest to the asker to migrate their question to another site (say, Stack Overflow for Git expertise). The asker can flag their own question to request it to be migrated. Moderators can migrate questions that are less than 60 days old.