Thinking about this, I wondered what it could mean, and the answer that came to me was that it was a reminder that the devil is much more intelligent than we are, so arguing with him would be a waste of time. Besides this, one would not be relying on God's help, but being lured into the trick of relying on one's own strength.
Later I picked up a book by William Ullathorne, OSB called Patience and Humilty (Sophia Institute Press, 1998) and was surprised to read in the first few pages something which illumines that fresco and makes clear the Benedictine wisdom common to the artist and the author. It was this reflection:
"It is the fostering of minor troubles until they swell to a flood of sadness and discouragement that gives the devil a turbid pool in which to cast his nets. If those minor troubles befall you, let them drop. Be not disturbed; turn your heart to God. Do not look at them; do not dispute with them; answer them not a word. Only turn your mind from them and let them drop."
Just like the fresco at Subiaco and the wisdom of Don Ullathorne were saying, one can not overcome one's troubles by becoming troubled over them, but rather by turning one's gaze to God, trusting that He is a Father Who loves and will give the grace needed for us to trust and wait in patience for His Peace.