while the THS was still moving towards 'full-up,' the PF applied full nosedown for quite a long period; probably because, as the report says, he was (correctly) countering the zoom climb. But the THS just continued on it's 'merry' way to full up
Where do you see any evidence of "full nosedown for quite a long period"?
During the transition of the THS there are only a few intermittent nose-down spikes - the trace below the zero line means the stick was held back, pitching up.
Check the elevator trace on the same page: once the THS is on its "merry way to full up", at no time is the elevator pushed to pitch down. Not even close. The best it gets is about 15deg, pitch up. No wonder the THS never came back.