I find it very hard to imagine that scenario.
From the moment the captain realised they were hot and high he would have been very aware of the energy state of the a/c. He would have been constantly monitoring thrust and airspeed and rate of descent in order to bring things back into line. I just can't imagine a distraction big enough to draw his attention from this basic and critical task.
If the rad alt was givig them false info maybe some other instruments were too. Hope so because otherwise it looks terrible. Subtle incapacitation????