Maybe they are testing a torpedo under a different codename.

It sounds like it is a good project but for a civilian purpose, anything other than that or a torpedo. Or for a false positive.
I cant think of another military use because the whole ocean will know of their position.

it wont matter who hears it, what are they going to do about it, it will be faster than any steerable torpedo, and faster tha just about ant thing on the water.
and anything that can go that fast on the surface would never be able to track it through there own noise.
Also the places it will be used they wont know what it is, it will sound like a speed boat.

The original article says it will used to deliver high value cargoe, what ever that may be.
I suspect the first application will be something like a high speed under water landing craft to insert a squad of troops very quickly.
It will herald the begining of a new era of underwater warfare, one where ultra silence wont be the end all be all.
If the demonstrator works, then expect to a whole host of new types of under water vehicles.
About 10-12 years ago i read a fascinating article on a naval engineering groups concept for underwater "flying" vehicles. Vehicles that would perform in much the same manner as an airplane, at under water speeds approaching 200mph. At that time the only thing holding them back was a suitable power plant.
A couple of years later i read another very good article of super cavitating projectiles/vehicles, two things of note was the mention of a sea water/powdered aluminum fueled turbine or rocket.
Essentially sea water is injected into a "swirl chamber" at high velocities, it is mixed with the powder aluminum, and an extremely exothermic reaction takes place creating super hi temp steam and aluminum oxide as an exhaust.
The steam is at temps like 800 degrees or something.
The high pressure steam cam be used to drive a turbine to turn a shaft or an electric generator or it can be just exhausted much like a rocket motor.
The steam can also be used to generate the gas envelope.
Just think about the possibilities of squadrons of under water fighters, shooting super cavitating projectiles from under water cannons.
Or inserting squads of troops onto a beach or into a port with almost no time to react.
researchers have fired super cavitating projectiles from an underwater cannon at muzzle velocities approching 1200'/sec, thats damm fast underwater.

along the lines rapid insertions
The marines are looking at orbital insertions of squads of marines anywhere in the world in two hrs.