They're saying that when the South Korean ship immediately fired after the initial explosion, the target that was identified by radar was actually a "flock of birds", or so they think.
Radar wouldn't be able to identify a submarine (or a mine), and neither would American satellites.
As soon as a salvage crew arrives on scene and establishes visual contact of the sunken ship, they'll know within seconds what sunk her.
Thanks for your comments on Post 29 - makes sense...