i think they meant more the area where the core and the center stalk touches. Those 2 are rubbing against each other pretty much all the time, so lubing that area can make some difference, although lots of other places an be lubed as well, like between the inside of the center stalk and the screw, inside the core where the screw is screwed into
but i think "lubing the core' refers more to b/w core and center stalk (bottom of center stalk)