LOL. C'mon man, you're the only one here who thought this was some kind of super secret capability. My entire premise is that this was a logically foregone conclusion based on what we know of their decades-old systems and the laws of physics with respect to how sound travels in water.

As for why they revealed their foreknowledge now? I don't speak for the military, but admitting to it vaguely, well after the fact is a significantly different situation than talking about it at the time and making it actionable intelligence. Furthermore, something may have leaked to a journalist, who then pressed them to make some kind of answer.

After debris was found, had Elon Musk twitter "SpaceX detected the original implosion at the debris site".

What conclusions would you draw?