Being a trebuchet, it will have to turn to attack, so it needs a move SKILL, but no command. This should explain nicely:
If there's a move skill, a hold position command, but no move command: Unit can rotate to face the foe, but cannot move. Example: Military's Howitzer.
If there's just a hold position command, it cannot rotate, nor can move. It can still attack, but the attack will come from the center of the model and face the opponent always. Example: Defense Tower.
The hold position command has one MAJOR fault though: You cannot choose a target, thus, your defensive structure are totally AI dependent, and I hate that part of it, but there's no way at all to use a regular attack skill without the unit moving, even with zero move speed.