As lovely as this page is (and Janson was a fine comic artist in his own right), when the difference in selling price for a Miller and a Janson page can be in the thousands (or tens of thousands) of dollars, then full disclosure is important.
If you're paying for a Miller, you should be confident that you're getting a Miller.
No different than if you're buying fine art. If it's being represented as a work by a specific artist, then you expect that artist's brush to have touched the canvas, and not his/her assistant or intern.
I'm blanking right now, but I recall allegations that there was at least one surrealist artist who, in his later years, had assistants do his artwork and he simply signed the finished pieces. Those later pieces now sell for a fraction of his authenticated artwork, and rightly so.