I think the statement is pointing out that even though an application (a node) is generating bursty traffic. Because this traffic is being fed into the leaky bucket algorithm, it doesn't saturate the network as the leaky bucket only permits a specified number of packets to enter the subnet at a given time. Thus, giving the application the ability to generate bursty traffic, as it can generate bursty traffic but the leaky bucket algorithm slows it down when entering the network.