Hatzerim,
Just FYI, 80 tons cargo is not too far from max cargo so even if there was someone else on the aircraft, getting back to where the fire was (presuming you could even find it) would be difficult - at best. A fully loaded MD-11 does not leave much room to go for a stroll.
Also, on this flight, there happened to be two crew. Sometimes there are more. Loadmasters are "typically" used to load the freight but may not travel with the aircraft for operators like Lufthansa Cargo. That said, sometimes additional crew are on-board, but I think you are being overly optimistic to think that someone is going to be able to squeeze by all of those pallets and containers to fight a fire aft of the wing, presuming that is where it started of course.
The guys did the right thing. Land Immediately. Fortunately, they were in a position to do that and not somewhere out over the Atlantic. If that were the case, we would possibly be telling a different story.