As far as I know, FDRs use magnetic media which only becomes demagnetised through extreme thermal cycles, mechanical vibration or shock and external magnetic fields.
I once worked on a demagnetiser for television video tapes, and those machines use a lot of energy and intense magnetic fields to wipe the tapes.
So, since these FDRs have been static and immobile at quite low temperatures on a mountain; I would have thought that there would be a reasonable chance of data recovery?