Obtain a private URL to a file in iCloud Drive without making the file available to anyone with the link
You're doing good except for the last line where you will have to share the link with someone. 

Share the link with yourself.

Choose imessages as the sharing option and enter your email or number. And add yourself.

To check the validity, This is a .Pages file and please try to access it and tell me if you can open and edit it or not.

Since this requires sign-in, I don't think you will be allowed to edit or even view it.

Only bad thing is the file name is in the URL