they may not engage enemy units with fire
My gaming group takes that to mean that they can't fire, *period*. I.e. if fired on, they can't participate in a round of fire except to roll their defense dice or, if they have not yet reacted, may attempt to move away / out of line of fire. If they get shaken during a round of fire, it's applied immediately - so if they lost the reaction roll and would fire second, tough luck, no firing back.
Is that the intent of this rule?