That's because the card forgot one eensy-weensy, insignificant, minor little thing (joke!)... the description of Rapid Shot (listed in its special abilities):
Rapid Shot
A model with the Rapid Shot ability can make one or more additional ranged attacks if the first one hits and injures the enemy, similar to Combo Attack, requiring a
wound with the first attack.
Any ranged weapon the model has in its profile that does not require reloading can potentially fire with Rapid Shot. A model with Rapid Shot and Fast Load is a dangerous character.
If the first shot hits and causes a wound, the attacker can use another action point to attack again, gaining an advantage if the defender is fallen, transfixed or dazed, or fire at an entirely different target. Up to three shots may be made using Rapid Shot.
Makes the Mantaform a beast indeed!