they add abrasive mixes to water and the pressure applied is continuous. They don't just blast an aluminum bubble filled with water against the cutting object at high speed.
And guess who didn't read the article which mentions that the jets works without abrasives as well? To quote the very first sentence:
A water jet cutter, also known as a waterjet,[1] is a tool capable of slicing into metal or other materials (such as granite) using a jet of water at high velocity and pressure, or a mixture of water and an abrasive substance.:
It's the kinetic energy that gives the jet its primary cutting power, the abrasives just make it more effective on certain materials.