Hot or cold it wont matter. It will rehydrate faster in hot than cold water thats all.
Polyclar simply does not do its job until it is rehydrated. It will re-hydrate in hot water, cold water, or in the beer itself. The instructions on craftbrewer get you the best results in the least time. Add stirred, re-hydrated polyclar to an ice cold keg and by the time you have sealed it up and given it a shake, the polyclar has already done its job and is ready to filter out. Filter cold and give it a bit of the Ross method for carbonation and you could be drinking crystal clear chill haze free beer within an hour of it coming out of the primary fermenter.
Or you can toss the stuff into your secondary/bright keg dry, rack beer (cold or warm) ontop of it and leave it overnight - more or less the same result. Or throw it into your primary fermenter hydrated or not (note, yeast effects the performance of polyclar, so unless its quite clear you might well need more of the stuff to get the same result) and stir it gently into the top layer. It has to mix through and contact all the liquid... So techniques where you can give things a good stir are more sure fire.
It all works - the technique on craftbrewer is the "optimum" and the further you depart from that, the longer it takes and perhaps the higher dose of polyclar you need to get the job done. But it does indeed still work.