The boy judged it would be easy to dodge cannonballs that flew in a straight line after looking at their trajectory, but this naive thought didn’t survive being put to the test.
Even after being fired, the shells are still Enkidu’s arcana.
They may not be able to make full turns, but they still could take tricky trajectories that defied the laws of physics. The boy immediately determined this was the case and chose not to dodge but to completely neutralize the projectiles with his defenses.

Fate/Strange Fake, Volume 7; Chapter 21 (Beings Shaped In Man's Image)