I can set up my own independent exchange to trade, say, gold in the same way there are thousands of online gold exchanges. Then I can artificially inflate the gold prices on my exchange, using fictitious money (like the willy bot did), leading customers to come to me to trade their gold for high prices, and therefore impacting the worldwide market price of gold... at least a tiny little bit for a tiny little while... until customers find out that they can't withdraw their fictitious money from my exchange, so my sham and its impact ends right there. This is what happened to MtGox in February 2014.
The bigger the market you are attacking is, the less time it takes for the sham to be discovered, because the tiny percentage of people who try to benefit from the fictitious high prices quickly overwhelm your ability to fake solvency. A huge market like gold would pretty much suffer no effect due to its size. And as the Bitcoin market continues to grow, the hypothetical impact of such a sham should diminish over time.