GetDlgCtrlID for top-level window with menu bar – return value

It is not inaccurate. You create a top-level window with CreateWindowEx(). Which looks like this:

HWND WINAPI CreateWindowEx(
  _In_      DWORD dwExStyle,
  _In_opt_  LPCTSTR lpClassName,
  _In_opt_  LPCTSTR lpWindowName,
  _In_      DWORD dwStyle,
  _In_      int x,
  _In_      int y,
  _In_      int nWidth,
  _In_      int nHeight,
  _In_opt_  HWND hWndParent,
  _In_opt_  HMENU hMenu,
  _In_opt_  HINSTANCE hInstance,
  _In_opt_  LPVOID lpParam
);

Note how you don’t specify the ID anywhere. But the fine print is in the description for the hMenu argument:

A handle to a menu, or specifies a child-window identifier, depending on the window style. For an overlapped or pop-up window, hMenu identifies the menu to be used with the window; it can be NULL if the class menu is to be used. For a child window, hMenu specifies the child-window identifier, an integer value used by a dialog box control to notify its parent about events. The application determines the child-window identifier; it must be unique for all child windows with the same parent window.

So you can have a menu OR a child ID. Overloaded, pretty common in the winapi, a child control can’t have a menu and a toplevel window can’t have a child ID. If you forge ahead and ignore this and call GetDlgCtrlID() on a toplevel window anyway then you will get back the value of the hMenu argument you specified in the create call. Well, today, always follow the api or you might get a rude surprise some day, you’d of course use GetMenu() instead.