I'm familiar with mach buffet which I guess some people call high speed stall. It isn't a stall but supersonic flow over the wing when you are high and too fast. Low speed buffet at high altitude is when you get supersonic flow over the wing because of high AOA and the increased airflow over the wing. Flying the big jets explains it very well if anybody is interested. The book explains a lot that most instructors don't know. Things like why the IAS for stall increases with altitude. Never found the answer to that anywhere else.