There's no evidence to suggest that the handling pilot of this particular 604 did not in fact use increased V speeds and/or a slower rate of rotation as suggested above. For all any of us know, he did apply both techniques.
People suggesting increased V speeds need to try to give a balanced point of view and not lead less experienced pilots into believing that when a wing is contaminted, increased V speeds are somehow a magic fix. Granted speed is good but only when you know what you are doing with it.
Increased V speeds techniique is available in the AFM and these are there to increase the climb gradient for obstacle clearance NOT contaminated wings.
From memory, the severe icing checklist on a 604 suggests that the MINIMUM speed that should be flown if Severe Icing is encountered is Vref + 27 and if you experience severe icing you would want to definately fly a higher speed than the minimum and this figure is based on Wing Anti Ice ON and working and is actually to give the stabilizer enough bernoullis to do its job as it is not fitted with Anti Icing capability.
So if you EVER have any doubts about contaminated wings and Hold over times, go back and get de-iced again!!!! DON'T USE HIGHER ROTATION SPEEDS AS A DEFINATE SOLUTION as there is NO evidence to suggest that an extra 10 kts, 20 kts or even 30 kts is going to do anything for you with regards to safetly getting airbourne with wing contamination suspected.
All the best
GW