I can see your local web servers

I started a local webserver listening on localhost:80 just to see what happens, but this thing seems to not detect it. It shows me "Scanning localhost ... localhost complete."

Edit: My guess is that this thing can only detect servers that send a CORS header that permits cross domain access.

It could probably do way better detection if it did not do xhr requests but added script/css/whatever elements to its own page pointing to localhost and detects if those error out.

CORS is a security mechanism for browsers to prevent leaking user information (e.g. cookies) when doing cross domain requests from a browser. CORS does not prevent accessing the server at all. You can always curl a CORS protected server but you won't be able to make a requests including the user's cookies from a disallowed domain.