Barbiesboyfriend, generally you are right if you want to maintain your speed.
However, as I'm sure you remember, advancing the condition levers is also used to increase your drag if you are too high or fast and you wouldn't want or need to increase power. Not the case in this instance but if somebody's SA is out the window it could just be another hole in the swiss cheese lining up.
Some Q400's have a Reduced Np setting. Basically you press a button and then advance the condition levers. The Propeller rpm is latched at cruise setting (850 rpm) until the power levers are advanced past a certain angle for the case of a go around.
I may have missed it but does anyone know if they were using Reduced Np on this approach? If they were, advancing the CL's would have had no effect on their speed.