Yes, they are sailing from both. if the cruise says Venice for embarkation, then the ship is in Marghera. You check in on Venice proper and they tender you and your luggage to the ship to get on and off. If your embarkation says Trieste (Venice), then it's not Venice at all. We are leaving out of Venice in October.
See this link about the ports and when you click on them, they tell you embarkation instructions for the port.
https://www.msccruisesusa.com/cruise/destinations/mediterranean