This really makes me wonder. How does it increase the gap, in comparison to B&W pictures? How is the difference not there already? And why is that bad?
So I checked the source of that quote, and there is no argumentation. None.
And (IMO), the whole point of "They shall not Grow Old" is that something like WW1 can happen again, and you, yes, you personally, will suffer if it does. Everyone has to realize that the people in that old footage are just like you and me, and Peter Jackson's film manages to effectively bridge the gap that blurry, jittery, B&W footage has, precisely by making it look recent.