That Limitation is similar to many aircraft with two AFCS systems....the airspeed limit might vary but the concept is the same.
In the "A" model with the "sharp" ended Aft Pylon...when you lost SAS the Yaw Divergence could be extreme particularly if in a turn and out of trim to some degree. The subsequent Models with the "Blunt" end to the Aft Pylon were much more docile but still required some attention when SAS was lost.
Supposedly, the Squared off Aft Pylon gave the aerodynamic effect of 31 feet of Tail Fin in yaw stability. It was very noticeably different than the A Model.
The Bell 412 for instance, has a similar limitation re AFCS.
In the early A Models of the Chinook, it was not all that uncommon to hear the Droop Stops pound when "skidding" after the SAS was turned off with no Warning. Usually, an Instructor only did that once in his career and afterwards there was plenty of warning to the Student. Few of us were really Suicidal or that Masochistic to do it on purpose after a really good fright the first time.