What I don't get is where are his companions? The acting CEO of Apple travels alone and carries his own luggage? Maybe the guy next to him is an interpreter, but still, Cook has to carry his own luggage? I bet Steve doesn't carry his own luggage.
The article states there are 7-8 people in his entourage, so they're probably out of the frame, perhaps behind him which would be the normal place for subordinates in terms of typical business protocol.
Tim's probably carrying his own briefcase -- not luggage, which would remain in the limo. He's probably pragmatic enough to believe that he'll know if he has his stuff if it's in his hands. Give it to someone else, and he'll be worrying about it more, not less.
Another possibility is the bag contains a gift for Cook's peer at China Mobile. Again, standard protocol would be for Cook to give the gift directly to that person, rather than have it handed to him from some lackey.