I've heard that LP pads have cerium in them. Does that mean I don't need to use cerium?

Unfortunately, even though the LP pads do have cerium embedded in them, you'll still have to use cerium to achieve your polish.

The cerium in the LP pads is used primarily as a structural bonding agent to keep the material together. On its own, it will not perform any polishing. It still must be used with a cerium slurry.

The cerium is used as a bonding agent so as the pad wears down it will not cause any unwanted scratches in your piece from more abrasive bonding agents (such as aluminum oxide).