The bearing is not sealed. I assume the cavity that the axle passes through the oil pan in is filled or at least exposed to the differential fluid, which would mean the bearing is as well up to the axle seal itself. I had leakage there and it was gear oil. They also have an o-ring where the carrier mounts to the oil pan which can leak.
I have to admit, when I do the volume math in my head and the differential spec says 700mil to fill, including the shaft cavity through the pan that doesn't sound like much.