This is a very good point and should be considered when deciding the focus of this proposed group and the kind of questions that should be asked in it.
In my opinion there is a need for a separate group, even if some aspects of web accessibility are covered elsewhere.
- Some types questions are not covered elsewhere. For example, how to test web accessibility (one proposed question asks what tools to use), or how to interpret a certain WCAG checkpoint, or maybe even some discussion if certain common accessibility guidelines should be changed, etc. You could call these "accessibility meta questions"
- As an accessibility consultant myself I would prefer a separate group concentrating on discussion about Web Accessibility, rather having a few questions in User Experience, and a few other questions in Stack Overflow, and maybe some others elsewhere.
You do have a point though, and this should be considered when selecting questions for this proposed group. I should probably go back and change some of the questions I suggested based on your comment...
Anyway, in my opinion there is a need for this separate group, and I hope it will get the required support form the community to move to the next phase