Customisation system: :type is not checked

[dieter@duenenhof-wilhelm.de (H. Dieter Wilhelm)]

Hello Emacs-26.3

Maybe I'm reading too much from (info "(elisp) Simple Types")?

There the manual says that the customisation widget provides
completions, which is working.  But it also says that the example
defcustom below

(defcustom apdl-ansys-launcher nil
  "This string variable stores the Ansys launcher executable.
When the respective executable is not in your search path, you
...
this for the current session only."
  :type '(file :must-match t)
  :group 'APDL-initialise)


MUST be a file and should even match an existing file.  But this
statement is not checked from the widget!  I can set the value to any
string.  What am I doing wrong or is my disappointment justified?

Thank you

      Dieter
-- 
Best wishes
H. Dieter Wilhelm
Zwingenberg, Germany