I think that there is some truth to this statement - and at the same time it serves to illustrate why we need to use complementary currencies.If a conventional currency turns enough times in a local economy, it has much the same effect as a complimentary currency.
Local circulation is indeed the main purpose of complementary currency. This is best exemplified by a type of currency that is called 'Regio', mainly implemented in Germany. There, the local currency is a cash-based extension of the national currency. People by intention can only spend it locally, and participating businesses in turn can only use it to source goods and services locally. The consumer who intends to 'buy local' will find it is easy to do. It might be more challenging for participating businesses, as they consciously have to 'buy local', too. It might not matter much what coins and pieces of paper currency look like, but if they look differently, they are a constant reminder of our commitment to local economy and community. Something we can easily forget when we go shopping in our 'local' supermarket.
As for other types of complementary currency, like LETS/Green Dollars/Timebanks, even more ties to the local community come into play. A cash-based currency is still a commodity, issued by an organisation. Green Dollars however are obligations and commitments between people in a community. They resemble a relationship, and the 'promises' behind those relationships are backed by the people who participate. A promise from one person to other people within a community is something very personal. This is entirely different from an anonymous piece of commodity that is handed from person to person, signifying some sort of 'value'.
While it is absolutely correct that if national money circulating 10 times in the local community would do the job, we are not aware of how pervasive the leaks in our local economy really are. A complementary currency will help us to be aware, it will also help us remedy the situation.