That is what the mini sub was theorized to be... A special forces multipurpose sub ideally suited to tapping or cutting cables.
That’s my suspicion. It would be a great way to disrupt the an enemy’s communications.
Another rumor is that it was trying to recover (in whole or in parts) a Japanese F-35 that crashed in that location.
The Soviets already have explosive charges placed on these transoceanic communication cables where they can blow them at the start of any war with NATO.