Probably has something to do with
supercavitation effect used in USSR's
Shkval torpedoes or improved variants thereof.
These torpedoes can do, as per Wikipeadia, only 370 km/h, which is somewhat faster that 100 m/s, i.e. more than three times
less than the speed of sound in the air.
Although I'm not an expert in this, the technology has likely improved since 80's - 90's when it was first deployed. However, if supersonic claim is true, I wouldn't expect any official information about it to be freely available in the open.
to come over here and tell us everything about this new technology (as soon as he clarifies the matter).