Read the captain’s narrative. He never says he intentionally lowered the nose. He says he was puzzled by the flap & speed indications & looked at the indicator & flap lever- then realized the aircraft was in a nose down attitude as he heard the GPWS. IOW, he was not conscious of the airplane’s attitude until it had become dangerous.

What he did do to address the airspeed issue was to reduce the power. Obviously, without compensating with elevator pressure, this will cause the pitch angle to naturally come down. He also calls for flaps 5. Assuming he hasn’t made too drastic a power reduction, the aircraft would still be accelerating which, in IMC can create the sense of pitching up. Now you’ve got an airplane with a tendency toward pitching down & a distracted & stressed flying pilot who senses his aircraft is pitching up. It’s likely he would be applying forward yoke pressure while fixating on the flap lever without even knowing he was doing it. At any rate, changes in pitch, power, & airspeed in IMC will throw your vestibular system out of whack. Throw in a distraction that takes your eyes off the instruments & it isn’t surprising at all. To me the outcome fits the pilots’ accounts quite clearly.