Sorry, but I can't buy into that.
At the top of the climb, the airframe had nothing but potential energy available (height, fuel and engines) that was not enough.
I am not trying to guess the cause of this terrible event, but I am trying to say that cargo shift is not the culprit.
If an aft cg causes a pitch-up like that, there is is nothing that a pilot or flight dynamics that could induce a pitch down?