The point was that you didn't know the difference between where elliptic curve cryptography would be used and a directed acyclic graph. Also a ledger is by its nature a directed (one direction) acyclic (no cycles) graph of dependencies, it just depends how you visualize it. Just like instructions on a computer are in a sequence, they can be separated into graphs of dependencies which may be woven together.

Other crypto currencies are able to synchronize in a decentralized way with different miners mining each block. As far as I know IOTA's structure makes this impossible, even though when they created it everyone else had already done it.