Oh, wait. You probably want more than that. Well, as I said, they're both right. On the one hand, the rappel loop is designed to pull the harness together so the leg loops and waist belt work to support you. That can be important on very long rappels, or if you have to stand in your rappel halfway down for some reason.
But in climbing, it's always smart to be as conservative as possible. The guide thinks that, also. That's why he or she was right, too. Rappelling is a single-point failure exercise. If ANYTHING goes wrong, then the result is very, very bad. That's different from lead climbing, where, if you fall, you (theoretically) have a range of backups to save youmultiple placements, a belayer, belay anchor, and so on. So, for safety's sake, it's prudent to loop that big carabiner around as much harness material as you can, just to utterly eliminate any risk if the rappel loop is worn, chafed, or otherwise compromised. Heck, when I climb I often add to my waist belt a chest harness made of one-inch webbing that is attached to the harness, and the rappel 'biner goes through that AND the rappel loop. This way, I have two systems on line, a main one and a backup. It also ensures that I can't somehow tip upside-down, which is always a slight risk when using a waist harness alone.