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.
Russia has changed a lot in a recent decade and will continue to do so, sanctions might actually accelerate some inner developments that are long overdue.