He may have actually said, "Everything should be made as simple as possible, but not simpler," or "Everything should be as simple as it is, but not simpler," or something else, or more than one variation, or none of them at all. The comment is considered part of the continuum of debate on, or the refining of, Occam's razor and such thinking. Me, today, I'm mostly wondering whether I ever noticed that bit of paper before, and if not, how not.
In the context of her office door, it feels almost parental, with a touch of the desire to calm, perhaps, along with possibly a little "Now, children...." And of course it could be seen as a reminder for herself, as she goes in each day.
Plus it's nicely put. A nugget. Chew-on-able.