The fact that we have these low frequency listening arrays has been known publicly for a long time. More closely guarded are the analytical techniques.
It's likely that the watch team flagged the implosion as artificial in real time, but could not identify the cause until other pieces of info were aggregated. The found wreckage today likely confirmed their suspicion.
source: worked at the navy's listening post watch floor years ago.
May he receive the sympathy he has shown to others.