Centaurus
Quote: "High speed jet aircraft may suffer both high and low speed (stall) buffet. The high speed buffet is caused by flow
separation from the wings as occurs behind a shockwave at high altitudes and/or Mach numbers. The low speed
buffet is caused by the same airflow separation as the aircraft approaches the stall angle of attack."
Seems to me that the first sentence is not very well constructed. If you omit the word (stall) the rest makes good sense, and the second sentence refers specifically to high speed buffet with no mention of stall.
That said, the wording of the third sentence makes it clear that buffet is always associated with airflow separation over the rear of the airfoil, which in turn is caused by strong adverse pressure gradients over the upper surface. At high speed these adverse gradients come from the passage of the air through a shock wave; at low speed and high AOA they are associated with recovery from the very high suctions over the front of the airfoil that go with high lift. If the loss of lift as a result of these separations is widespread and sudden then one may get the classic stall pattern.
However, there is a complication in that many modern wing designs don't have classic 'stall' characteristics. According to FAR25 stall can be defined either by classical loss of control or by buffet sufficiently severe to make it nearly impossible to fly the aircraft properly. For the aircraft I am familiar with the separations spread gradually and 'stall' at high speed is defined by the onset of severe buffet. Even with flaps down this may also be true. [Formally, the buffet limit would be +/- 1g at the pilot]
In this sense then one might have something called high speed stall buffet, but it isn't the same sort of stall as, for example, a sudden wing drop.
Not sure if this is enough of an answer, but I am trying to avoid complications
