Actually, this is not true. Per page 12 of Revised rulebooks, under Phase 3: Combat Move, it states:
"Antiaircraft Guns: Antiaircraft
guns can never move in this phase;
they can make noncombat moves
only. However, they can make a
special attack in this phase only.
Whenever an air unit moves into
a hostile territory containing an
antiaircraft gun, the gun gets a
chance to shoot it down."
It clearly states it is a special attack IN THIS PHASE, meaning the Combat Move phase. So, triplea is programmed correctly for this rule.