They have yet to fully explain how their network is secured. It can never be secured by PoW alone, as very little hashing is being done. PoW only scales linearly with transaction rate. The critical pieces of their security are one or all of the following: 1. Restricted peering. 2. Network topology. 3. Bandwidth limitations.
Its not explained in their whitepaper, or in any follow up blog, or anywhere else.