Not so sure - Our Boeing FCTM suggests that using a block derate (not assumed temp...) to reduce Vmcg in contaminated runway conditions may raise the perf limited take off weight. This is probably within the 'certification conditions' that you note which gives the crews an option.
Perhaps you might re-read my post. I thought I had distinguished between flex and derate ? Your manual note looks to be pretty well what I was talking about ?
All's well, though, the value of these sorts of threads is to get people thinking and challenging their ideas so that, at the end of the day we all might move forward a little with our various knowledge bases.