But in this case with slow speed, it's the massive (literally) amount of mass of the cargo ship that gives it an un-intuitively large amount of energy.
All objects are surprisingly movable when they encounter enough mass.
To your point, it is mass. Most people don't understand mass on water. At 1 knot the ship would do the exact same damage, topple the support and drop the bridge.
Motor control and rudder control in a momentumous frame of reference.
sqrt of 100K times weight of a car is 300 cars worth.