Non-expert guess:
It may not be totally a distance thing as RF can be a funny (although predicatble for the experts) animal....
but 10' is not an extravagant distance for a USB cable. I use a couple of this length and do not seem to experience any data issues. If the bits get there OK, the music is not decoded until it gets into the reciever so the more susceptible analogue music signal picking up interference is not a factor.
I don't know enough about USB standards to say for sure.
I suspect this is a "just in case" disclaimer and that using a shorter cable would reduce the odds of any issues and especially if it is carefully routed away from the big power supplies in the Denon.
Also, I have seen music played from a portable hard drive into a 360. My drive and my Buddie's 360. It was plug and play!
Another option would be wireless networking and letting the PC act as the music server. However, because you will have the advantage of a receiver that has fancy, on-screen GUI built in to select the music, you don't really need the PC or the hassle of networking.
That's tech talk for "I will be jealous."
