Not such a good point, given that one indeed survived, and that in that particular case it wasn't the impact that destroyed the other one but the post crash fire.
Black Boxes are designed to stand 3,400 Gs what is about 300 Mph. Maybe the airplane did hit the ground at close to those speeds.
And the fact that the second one did not survive the crash due to fire, shows they are not indestructible. Fire is actually what they are supposed to survive for at least 1 hour.
This is a very simplistic view of an airplane crash. The whole airplane is the crumple zone for the black boxes mounted in the tail of the aircraft, that's for a pretty good reason so there goes one assumption, it's not like they are in the nose. And for another, those black boxes that were lost usually were lost either because the whole airplane was lost or because of the post impact fire.
There are some instances of flight data recorders and cockpit voice recorders that were destroyed but not a whole lot of those. That said, this was an exceptionally violent impact and it remains to be seen if there is anything usable.
But - again - speculation is utterly pointless there is simply no information to go on other than the angle and speed of impact, which definitely isn't giving hope, but it doesn't rule anything out either. So let's just be patient until there is actual data. Nobody ever said that black boxes are indestructible.
And the fact that the second one did not survive the crash due to fire, shows they are not indestructible. Fire is actually what they are supposed to survive for at least 1 hour.