The drain plug calculations above assume that there is only atmospheric pressure under the drain plug (which balances out the atmospheric pressure above the water above the plug), so the force holding the drain plug down is only the difference in pressure above and below the plug acting on the horizontal surface area of the plug plus the submerged self-weight of the plug.  If the plug were not sealed to the drain line so that that there was a lower water pressure below the plug, then only the submerged weight of the plug would be required to lift it.