What did Dogen think a miracle was?
As Chogyam Trungpa explained in Shambhala: The Sacred Path of the Warrior (or was it in The Myth of Freedom and the Way of Meditation ?), the perfectly enlightened Bodhisattva always does exactly what's right at the moment. The perfectly enlightened Bodhisattva is always on time.

Because their mind is not running ahead nor falling behind, it is always exactly so. Being exactly so, it is without conflict, and is perfectly in sync with the needs of the moment. It is also 100% authentic without pretense, as simple or as complex as presently required.

This is what Dogen said Guishan praised as "miracles" and "no small matter".