They claim the speed of sound in water in the article.
Looking that up, it’s 4748 fps at 50F and that’s pretty damned fast. Speed of sound in air for comparison is 1100 fps.
Figure it from the perspective of the energy requirements to open a corridor in water for passage. The energy requirements go up a cubic curve relative to velocity. Maximum energy available is a limiting propulsion factor. At sufficient velocity the energy dissipated obviates the need for a warhead.
The dynamic pressure to the leading structure inflates the bubble and high pressure gas forms a barrier between water and the vessel hull. At some point the speed of sound within the gas will be exceeded by the vessel, and the barrier will fail. Cavitation damage results from fluid flow induced by the collapse of a gas bubble.
The nosecone of the Shakval was wetted and contained the seeker head. It also provided dynamic lift in combination with propulsion, to aid suspending the torpedo above the bottom of the gas cavity. Small attitude changes of the wetted contact area create large side thrust at speed. A minimal contact patch ahead of the main vessel bulk must be present for sensing, unless run blind with inertial guidance.