On the A320, alpha floor is not inhibited above M0.53. In this case, we could have alpha floor activation at FL390 (example). That is my understanding of the whole thing.
I done it on the simulator for the A330 and alpha floor does not activate whatsoever until the aircraft is at an altitute that allows flight at M0.53.
I'm sure the rate of deceleration does not play a role here, since the alpha floor trigger is AOA, and not a "projected time to reach alpha max based on current deceleration rate assuming you are gonna give further pitch up input when you reach it".
I found nothing so far written on the manuals that convinces me that alpha floor will not activate at cruise FL for the A320. If someone experienced this situation or could try it on the sim it would be great!