The reason is Apple. It is simply not possible to develop iPhone/iOS apps on any platform other than a Mac.
Ionic/Cordova/PhoneGap allow you to develop in HTML5, but when the app is actually built, it compiles to a native format, be it Android/iOS/whatever. Since you can't compile a native iOS app anywhere but Mac - it's a no-go on Windows, tho you can still do your HTML5 dev work there.
So that is what the docs mean. You can fire up an editor and write your Ionic app anywhere, but when it comes time to build - if you want to build for iOS, you will need a Mac.