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.