Uplinker
The captain was talking to ATC at this point, perhaps writing something down? Looking at his chart? (There appears to be minor confusion about whether they are on HDG or LNAV), could have been looking at navaids to adjust? Anyhow after the FO does yank the controls forward, the captain does respond by pulling up - but the FO countered throughout, right until the end when yelling from the jumpseat appeared to make him stop.
There is always startle factor etc, however that does not justify the fact that the causal factor of this incident was an incompetent pilot nosediving an aircraft to the ground. If I were the captain’s family I would want answers anyway.