Any recording equipment the military have in the area will be used to warn against hostile threats. The US Navy, understandably, wouldn't voluntarily want to divulge the location or capabilities of such equipment.
I read a report that said as soon as the Navy corroborated their recording with...
Classic chinese whispers.
The rescuers report underwater noise from an unknown source -> a talking head on the TV says "well, as an experienced submariner, Nargeolet would know that the drill is to knock on the hour and every 30 minutes after that" -> Media reports that knocking is being heard...
"Not gonna lie; I was a little nervous, especially given the paperwork, which read, 'This experimental vessel has not been approved or certified by any regulatory body, and could result in physical injury, emotional trauma, or death.'"
CBS article by a reporter who did the trip