You're right, economic growth does not depend on the money supply, but I believe the point being made was that it can be adversely affected or slowed by it. Deflation to anything less than a purely cash-based economy would have to make up the difference in economic activity with delayed payment or barter. Even reduction in available credit can act as a drag when the system has grown into such flexibility.

If a money supply remains constant while the economy grows, eventually the money supply becomes inadequate to facilitate trade throughout the entire economy. Consequences vary, but all are disruptive.