I read the quote to mean that it's silly to claim that a plan is perfect when it's actually unworkable.
Of note, Alfred von Schlieffen, the architect of the original deployment plan for war against France, was on record as recommending a negotiated peace in the event that the German Army fail to quickly draw the French into a decisive battle. Obviously, this recommendation was not followed. Also of note, Schlieffen's plan was explicitly for a one-front war; the bit with the Russians was hastily tacked on by Schlieffen's successors at the General Staff.
Another month, another rationality quotes thread. The rules are: