So, if one makes a poolish and let's it ferment in fridge for 18 - 24 hours, one would not also then cold ferment the dough is what you are saying? I suppose the pre fermentation does the same job as the long cold fermentation?
No, what I mean is that AFTER the poolish/preferment, it doesn't matter how you ferment your dough (CF or RTF), but you will need to ferment anyway, but it allows you to ferment for a shorter time.
When making a poolish, you will never be able to use 100% of the flour unlike a biga (poolish being a 100%-hydration preferment and all), so there's still the remaining part of the flour that will need to be fermented (actually, even a 100% biga will need fermentation anyway).
Besides, as the poolish is very liquid, it's a good environment for the yeast -and I guess, all kind of bacteria- to have a better activity = faster activity.
So you'll always need a fermentation after the 2nd phase, whether it's CF or RTF (in French, we call the preferment "1st phase" and the final mixing "2nd phase").