He is correct- they are sealed and are not recommended to be greased.
Some of us take the next step and do them since they can be greased with a tool you can buy aftermarket that allows you to pump grease in the bearings from the middle where it is open. you have to pull the hub, slide the adapter greaser tool on the shaft like it would be the hub then pump grease in the opening in the middle of the bearing. you have to be careful when you do it so you dont over grease it and push the dust seals on the outsides off.
Does greasing them help? maybe. i havent had trouble with any of them yet dont drive through a lot of mud either. we grease car hubs, axles, bearings etc. and almost nothing gets greased on these off road machines. i prefer to do as much greasing as i can, thus why i did my x3. i got lazy and didnt do my defender i suppose. it isnt just a quick thing to do, it is something you have to be committed to doing because you have to remove parts, torque stuff back down, etc.
steve