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.
I am well aware (knowledge bias).
FDRs are most frequently damaged in a crash and almost always returned to the lab.
I can’t recall ever reading such a statement in the wake of a crash (”The aircraft’s flight data recorder (FDR) was found to be partially damaged, making data extraction impossible on-site.”).
Maybe it was a translation thing, but unneeded statements of obvious facts set off my red flags.