So ..... once doors are opened for evacuation, is the aircraft no longer in flight ?
No! It says
right in the regulation:
...until the time at which any external door is opened for the purpose of disembarkation."
The aircraft had not reached its disembarkation point (the gate) when the evacuation commenced. Since the doors were opened for a purpose other than disembarkation, the aircraft is still considered in flight for the purposes of the Aeronautics Act.