Grity,
The sensitivity of the calculated updraft velocity to errors in AoA or pitch is evident from the formula given in the header of the graph. For example, for GS + HW of approx. 500 kt, the updraft velocity changes by 88 fpm for 0.1 deg error in the recorded AoA.
Quite another matter is the pitch response of the aircraft when traversing an updraft zone of increasing or decreasing vertical velocity. That effect exists and has been discussed on an earlier AF447 thread, but is smaller than you suggest: The updraft increases about 2000 fpm in 5 seconds, and 40 fpm / 500 kt is about 0.05 degrees. The actual effect is even smaller due to the downwash behind the wing and other damping effects.