Just prepping this for Adventurer's League and the wording is very odd in the description of the way in which the Siege Engines operate.

Taking the Clockwork Trebuchet as an example

Pulling a lever of the Clockwork Trebuchet requires the Utilize action. Then a creature can take the Trebuchet Stone action.

To my mind, the word Action is very specific here in that second sentence. A creature will use their action in a turn to pull the lever, loading and aiming the Trebuchet. To fire the Trebuchet is in itself another action, meaning that a single creature would need to be either Hasted or Action Surge to pull off both pulling the lever and firing the Trebuchet in a single round. 

Comparing the Adventure's siege equipment to those in the 2024 DMG, the equipment in the DMG is much clearer in that the action to load and the action to fire are clear and separate actions, either requiring additional turns or an additional character to complete the actions necessary.

I am confident this interpretation is correct but happy to be corrected

I see it the same way as you.

Utilize to pull the lever, and that enables a different action named Trebuchet Stone action.