I have same puzzle to solve..
I have a User Role set to edit, create, remove all posts of transparency (custom post type).
I have done this activeting “Force custom post types to use their own capabilities”.
To the same Role/User I have to let him edit 2 pages (classic wp pages).
If I add the ID’s on Edit Posts, he cannot modify anything but those 2 pages.

I don’t see any codes in your link above.
I need to operate in functions.php? Can you give me a hint?
Thank you