Watch from 3:50 on to get a better idea what we are discussing. When you see those little pieces of sting stand straight up and then reverse, that portion of the wing is stalled.
That stall progresses if the pilot continues to increase angle of attack to CLmax until the wing can not longer support the weight of the aircraft and is no longer flying.
That flow reversal over a portion of the wing is what causes aerodynamic stall buffeting.
At 4:29 the test pilot begins recording a "light" buffet. Observe the tufts and imagine a "heavy" buffet....
An airplane does not increase its turn rate if it experiences aerodynamic buffeting. An airplane that buffets will decrease it's turn rate when the buffeting begins.
When artificial means such as stick shaker were not available, the only choice a designer had was to reduce the amount of stall warning he gave the pilot. The less buffet, the closer the pilot can fly to CLmax without degrading his turn performance due to aerodynamic buffeting.