I determine a topic-ness of a subject in two separate parts:
- It's about Ubuntu or one of its derivatives or anything related to them (which is precisely the quote you mention in your question).
- It's about an official Canonical product.
The second part is backed by the following text in the help center about the questions that we can ask about:
Questions that you may ask:
- [...]
- Services provided by Ubuntu and Canonical.
- [...]
WSL falls in the second part, which is an official Canonical product. So I woulddo not vote to close as off-topic virtually anything regarding WSL. That also includes installing it on Windows. Whether it's better suited to be asked on a Windows-specific Q&A site is irrelevant in my opinion. Same reason as to why Ubuntu questions can valid on Ask Ubuntu and Unix & Linux. Same reason as to why Ubuntu questions can be valid on Ask Ubuntu and Unix & Linux.
This reasoning of mine falls back to when Canonical had the, now discontinued, cross-platform Ubuntu One sync product. Any questions about that product used to be on-topic such as How to install Ubuntu One Windows Beta on XP?.