737-200 and old DC-9's used to rotate at 6°/second interestingly.
My copy of the Boeing 737-200 FCTM 1975 states three degrees per second up to fifteen degrees. The Boeing Airliner magazine same era says same thing. Certainly Boeing never recommended more than three degrees per second when the 737 first came out.
I bet it was your chief pilot's personal technique rather than a manufacturerer. Sounds lethal to me.