The help center is actually clearer than that. You are quoting the on topic page which states (emphasis mine):
Unix and Linux Stack Exchange is for users of Linux, FreeBSD and other
Un*x-like operating systems.
If your question is about:
- Using or administering a *nix desktop or server
- The Unix foundation underlying OS X (but generally not frontend application questions)
- The underlying *nix OS on an embedded system or handheld device (e.g. an Android phone)
- Shell scripting
- Applications packaged in *nix distributions (note: being cross-platform does not disqualify)
- UNIX C API and System Interfaces ( within reason )
Those two first sentences in bold make it clear that anything *nix related is on topic. The rest are to explain the edge cases. Those systems which are kinda sorta *nix but not entirely. So we specify that for those systems, and those systems only, their *nix side is on topic here but not the rest.
"The underlying foundation of server and workstation Linux, BSD Unix, AIX, HP-UX, etc" are all covered by the first bullet point: any question about those systems will come under "using or administering a *nix desktop or server".