You are right. If they are indeed dead roots they are not giving nutrients to the plants (unless you want micro-organisms to have them decay, then there's a cycle...) , so trimming them of was the right idea. There is no living vascular tissue to transport anything to the plant.
Perhaps he meant that they are supporting the plant to keep them upright rather than loose in the pot?
I also have not heard of a cycle on tropical orchids, what are you growing? Perhaps there is a cycle on orchids like a Lycaste
A yellowing leaf is a different idea, sometimes they do support the plant while the energy stored in the leaf is moved into the main plant for use. I see that blooming plants have the bottom most leaf yellow sometimes.