(that radar failure ment he lost an instrument.)
The radio altimeter indication was not a failure, as in flagged invalid, but erroneous altitude with no indication it was invalid. That is quite different, and most disturbing.
Engineers at the airframes and component manufacturers go to great lengths to avoid erroneous indications. It's calculated to be on the order of ten to the minus ninth for Cat III autoland.
I'm not intending to excuse the crew for stalling the plane, but pointing out in this case the hazard of avionics with undetected failures.
GB