The PF probably intended to land with A/T off but was habitually conditioned to having A/T on (AC Rouge SOP strongly recommends A/T on from takeoff to landing). The PM probably thought A/T was on and did not monitor the speed.
How did we manage before autothrust was invented? The A320 handles just fine with all the automatics off. I'm not sure why the folks who write the SOP think otherwise.