The behaviour of throwing seems well known. It is only the degree of planning which is being questioned. As wild chimps know the geography of extensive areas including food sources, i dont really find this particularly surprising.

http://answers.yourdictionary.com/answers/animal-life/chimpanzees-protect-themselves.html - Some of the things that a chimpanzee might do to protect itself both in the wild and in captivity from other animals of the same kind or animals of a different species include:

Throwing sticks at a predator or something they view as a threat

Throwing stones at a predator or something they view as a threat

In the wild, chimps will escape to higher ground, such as treetops, where they are out of reach of major predators

Stay and fight; some chimpanzees who are protecting their family social groups will simply stay and fight to the death

Major Predators of Chimps

When do chimpanzees need such protection most? Like all animals in nature, the chimpanzee is threatened by a number of different predators. Their main predators in the natural environment are lions, tigers, cheetahs, and any other large cat also native to the environment where a chimpanzee typically lives.

It also seems that chimps can plan to make spears!

http://estebanrivas.wordpress.com/2011/10/01/great-apes-and-their-attitude-towards-other-animals/ Many different animals are being eaten as food by great apes. Chimpanzees hunt red colobus monkeys, vervet monkeys, baboons, bushbabies (where the chimpanzees make spears out of branches to impale the bushbabies in hollow trees), bush antelopes, bushbucks, warthogs, bushpigs and even human children.

Big cats are the natural enemies of great apes in the wild. Leopards hunt all four great ape species. Lions hunt for chimpanzees and tigers on Sumatra for orangutans. The chimpanzees and bonobos drive the big cats away by hurling sticks, branches and stones and using their intimidation displays. Famous are the experiments by the Dutch ethologist Adriaan Kortlandt, who put a stuffed leopard with a baby chimpanzee doll in its claws. The chimpanzees in reaction used sticks and branches as clubs and lots of aggressive behaviour towards the stuffed leopard.