The problem here is that limited overs cricket has been modified from regular test cricket in a way that fundamentally alters the strategy and flow of the game to increase the action and decrease how long it takes to play.

Because of those concessions to speed and entertainment, some rules that worked ok in test cricket break the game in limited overs. In this case, NZ was down to their “final over”, a concept that doesn’t exist in test cricket. Underarm bowling basically removes overs from the game and can guarantee a win for the bowlers, but in test cricket, would lead to a draw.