I think there's terminological confusion here.
There's a flight region (combination of IAS, mach, density altitude, etc.) at which imposing a 1.3g load will bring the aircraft "near" a stall (ignore the meaning of "near" for a second).
Above 20,000 feet, buffet is a reliable indicator that you're getting into that region.
Below 20,000 feet, buffet is not a reliable indicator, and so the ADC and other magical contraptions shake the stick when you're getting into that region.
The ADC and other magical contraptions also display the amber band such that, irrespective of whether you're at high altitude or low, the top of the amber band means you're getting into that region.
In other words, the top of the amber band always means the same thing: you're getting into that region. Above 20,000 feet or so, the actual aerodynamic physics of the aircraft (i.e. buffet) and the amber band will give you indication at about the same time. Below 20,000 feet or so, the stick shaker and the amber band will give you indication at about the same time.