For what airplane is that "Boeing" recommendation? In what document?
B757 & B767 and as far as I am aware, all Boeing types that have CWS installed.
Direct quote from the FCTM - Flight Crew Training Manual.
I have had to disconnect the autopilot only once because it could not hold attitude in [moderate] turbulence
...and that is exactly my point. Your autopilot (unless it was engaged in CWS) was not trying to maintain an "attitude" in the first place - it was trying to maintain a barometric pocket, ie: your altitude or flight level. In severe turbulence that is exactly what you want to try and avoid and why engaging CWS is so important as it will maintain an
attitude instead.