Actually that is not true. I can tell you first hand while they are called sealed they are not "sealed" The grease does "dry out" and water etc does get in. If you remove the antilock sensor from its port and friction fit a rig to grease thru it you will have grease coming out the seams without any real force pressure at all. If that viscous fluid can go out imagine how easy water etc can go in say in winter snow or water crossing etc. A number of guys are greasing the bearings and are getting serious life from the bearings.
The other thing is just like greasable ball joints while yes things can get in you actually have something you can do about it rather then just be stuck until they are worn to crap from it.
But even if we went on them being sealed you could still grease thru the port but stop at the any resistance and that would prevent it coming out. I ahve a theory but have not gotten around to trying to prove it. THe guys that use the vacuum hubs seem to go thru unit faster than those with no vacuum. I think something about that vacuum maybe effecting the usable life of the grease or possibly somehow pulling foreign matter in. Just something I noticed in reported mileage out of unit bearings.
I think the thread you saw Steve was from me.
Here is some of the one where I mention it and posted pics.
front hub bearings - PowerStrokeNation
PowerStrokeNation - View Single Post - Drivetrain vibration help
Now very important you need to clean and repack those needle bearings in the back of the unit bearings just as frequently.
Unless you are driving thru water that covers the hubs doing it all every 6 months would be my recommendation. Just pull the unit bearing completely off. It will only take you about 30 min per side. Clean and grease the needle bearing. Then CAREFULLY remove the sensor from the port. Easiest thing to do is find a hose that will fit tight in the port and then either with a fitting or needle end start pumping the grease. Rotate the flange 1/4 turn to evenly grease the bearings. (likely not needed but I think its better that way) Go unit the grease comes out the seams or not ( up to you). I prefer to as I have seen water come out with the grease. Then wipe clean off any grease form the outside of the hub/ If not it will get on the rotors and then watch out!!!! Reinstall them and you are GTG. Obviously this is also when you do your ball joints tie rod ends etc...
For grease did not use moly. Only reason I say not to is Timiken the manf does not. They use full synthetic Mobil One grease no moly. Figure why take a chance although looking at one broken down I can not see any thing it would hurt and may help but still. Only my opinion there no other proven basis.
In case some wonder about the effect on say the brake sensor its nothing. Its non conductive and there fore has no effect on the ABS sensor's perf.
Do not try to grease thru the vacuum port it goes to a differnt part of the unit and can also screw up the vacuum auto lock from working..
For more info do a search in this forum with my username and grease.
Also go over to Ford-trucks and to the superduty section and do some search on greasing unit bearings and you will see I am only one of many.
If you have a non ABS you can check the unit and you will many times find a plug in the port. Grease thru that the same and then reinstall.
One idea I may try as my units will go faster with 40" tires LOL I want to drill a hole in line with the sensor port but do it inverted to prevent any metal shavings going in. Then tap it and install a zerk fitting to grease it easy without having to pull the sensor. I think someone did it this but did not think of adding the fitting. It worked without issue.
BTW since I mentioned water and getting your hubs submerged. If you do this you should also drain your axle difs and refill. Those seals that look like hoolhoops about a month after you install them are not stopping any water from getting into the axle housing and only the inner seal is there after that with a rotating axle water has a great chance of making it thru. Just something to consider.