What he meant was that a diesel engine doesn't need to increase fuel rate just because RPM increased -- instead, it can keep using the same fuel rate and run leaner. You had asked if you should leave your diesel in gear or in neutral while descending, and I answered that diesels may not DFCO (I have no idea), and that older cars mostly wouldn't. He was saying that it may still use fuel at the same rate as idle even though your RPM is up.
I just realized that I failed to give even as complete an answer as I could. Even if it doesn't use more fuel to descend in gear than it does to descend in neutral, it does slow your car (or prevent it from accelerating as much). If you use neutral, you gain speed (or fail to lose speed, or lose less speed), which means you won't have to step on the accelerator again as soon.
Don't worry about it, you've completed the picture. My question is now completely clarified. Thanks a lot.