At apogee, the aircraft had no influence on her flight path. She was Nose high, AoA thirty degrees, and climbing, and her flight path was "level" for a brief millisecond, before she started falling at 1G. She started rolling, and was accelerating quickly toward her reported maximum ROD of 15000 fpm.
You're taking a few liberties with the flight mechanics. At apogee the flight path is level, not climbing nor descending. The AoA is equal to the pitch attitude of about 12.5 degrees. The normal acceleration is about 0.75 g as the vertical speed reduces from 1500 fpm positive through zero at apogee towards ROD of 15000 fpm 43 seconds later.
Nose down past this point did nothing to move the THS off the max NU position.
Yes, the nose-down inputs were too small and too brief to change the position of the THS.