Over on Photography, we not uncommonly get what sounds like a very similar situation around graphical editing tools (Photoshop etc): "Photoshop is used by photographers, therefore my question about Photoshop is on-topic".
How we deal with this is exactly how you'd expect: if the question is actually about photography (e.g. "How do I use Photoshop to remove a blemish from a photo?"), it's on-topic. If it's not about photography (e.g. "How do I use Photoshop to draw a picture of an elephant?"), we close it with a custom close reason phrased something like "Questions about Photoshop are on-topic only if they have a clear photographic application. Please edit your question to include the photographic context" and/or pointing them to an appropriate site. I'd imagine something similar could work here:
Questions about open source software are on-topic only if they relate to the open source aspects of the software, as opposed to using the software. Such questions can potentially be asked at Super User or Stack Overflow.