This is based on my approximation of the movie's time setting - I would wager about 1500-1600 AD.
Remember Vizzini's battle of wits? He claimed Iocaine Powder is from Australia, which is peopled with criminals - who are not used to being trusted.
Australia was not founded as a penal colony until late 1700's.
How am I supposed to believe any of Vizzini's other pearls of wisdom if he's so wrong on this one? Maybe it is actually okay to get involved in a land war in Asia.....