Perhaps it reflects the way popular textbooks simplify a complex subject by looking at one aspect in one chapter, and at the other in another.
At progressively higher altitudes, the window of safe airspeeds between these two phenomena gets progressively smaller."
Yes, and at a certain altitude (popularly known as "coffin corner") the window disappears altogether. The 'low IAS' is then about 0.8 Mach, the pre-stall buffet due to high angle of attack is then buffet due to high Mach number.