AOA measurement accuracy and its closeness to stall is all related to Mach no.
At increasing Mach no, max AOA decreases.
AFAIK, above FL250, a Mach No equivalent to safe KIAS, may be too much giving a reliable AOA measurement. So as GR mentions, Airbus may not be comfortable flying simply as relying on AOA measurement (BUSS) above FL250.
In fact, this may be an explanation of Vls at high altitude. Although Vls leaves a margin to Vs1g at low level (1.23 x Vs1g at clean config for A 330), it changes to display margin (around 1.3 g) to buffet rather than to Vs1g at high altitude.