The coefficient of friction between a highly polished hardened metal sphere and the plastic of the diff gear, with a bit of silicone grease, is lower than metal on metal with a viscous oil.
Going from the link you posted, the normal force in this case would be the transfer of torque from the diff gear to the diff out-drives through the diff balls. The more torque, the more force, the more force from friction that resists diffing action. The instant one wheel looses traction, the torque drops and so does the friction. That's what ball diffs excel at: instantly letting diff action take place if one tire goes over a pothole or over a slick spot, preventing all the torque from being applied to the other tire and causing it to break traction.
We aren't saying this friction that 'locks up' a ball diff doesn't exist. Just that it does so to a lesser degree than a typical spider gear diff with common diff oils. Another part of this is the location of the diff balls relative to the center of the diff. The further away from the center of the diff, the less force there is on the components to transfer the same amount of torque. The balls in a ball diff are further away than the faces of the gears in the gear diffs by a good 30-40%. This reduces the force (normal force, to tie this into the previous paragraph) which reduces the friction force.