Utter tripe! DES NOW, ALT INTV etc are features to be utilised. For descent they allow VNAV to manage a capture of the geometric profile when in PATH or if ATC requires you to DESCEND NOW. Gives you 1000fpm until TOD is reached then, as you say, idle thrust and pitch to maintain path. In the interest of CRM say nothing - it isn't a safety concern. For your knowledge
A) if 'engine cooling' was such a concern then why develop such a function?
B) Not all descents are initially continuous, so thrust and EGTs will vary ie in a step descent, descent at a required V/S or descent with an intermediate off.
C) We live in the age of the modern efficient turbofan. Not Frank Whittle's suck, squeeze, bang, blower!
You hear some tosh on the line. Just nod in agreeance and play to her rules on this one. It isn't worth debating.