That makes no sense.
Flight data recorders are robust but have weaknesses. As one might expect, their power and input/output connections are especially vulnerable to crash damage. That is the most likely problem, so the NTSB lab will carefully open or even disassemble the box, connect directly to the recorder inside, and pull the data.
It makes perfect sense.
One or more Deep States want to control the narrative.
An electrical connector was broken off the FDR. They had no means to fix that in Korea under the controlled circumstances required to preserve the data evidence chain..
I think it does make sense. Probably the NTSB has specialized equipment that the South Koreans simply don't have.
When I was in the Air Force in the 1970s one day a lieutenant came into our instrument shop with an Angle of Attack indicator (AOA) that he said was removed from an F-4 that had crashed, and he wanted me to tell him if it was working at the time of the crash. I hooked it up to our mockup and it was no longer working. Since indicators normally return to zero with spring action when power is removed, I told him my guess was that it was working at the time of the crash but I thought it really needed to be opened up by the NTSB for a better answer.
It was indicating an AOA of -2. I don't think a wing would produce much lift at all with an AOA of -2.