Anything can happen. It is true that it didn't magically open on it's own, but the signal coming from Alonso's car could have been corrupted, there could have been a software error on the FIA side, there could also be a bug in the MECU. The button Alonso pushes may have had a problem. Alonso likely did press the button, but it within the realm of possibility that he didn't.
Point is that nobody know's equivocally what happened, unless you know something we don't.
A lot of it depends on how the system works. Is the car sitting there in listen mode, waiting for the FIA to change a flag from 0 to 1? Is the car always aware if it is in the zone or not? Does the FIA send just one flag saying it's time when the zone starts, or does it also send one when the zone ends? How the logic works would be the piece of knowledge you would need to know he pushed it. Every time that the driver pushes the button, does it check with the FIA system to see if it is ok, or does the car already know that it is in the zone? How much of this logic takes place in the ECU, and how much in the FIA system?