> A smart contract is simply an agreement between parties, written in codes and enforced by a third-party.
No, it is an agreement between parties enforced by cryptography, and no third party required.
> In the case of Ethereum, it is enforced by cryptography.
No it is enforced by payment to the network, just like aws, but in a worst way, because code is executed wastefully, while still remaining at a central location.
What you are not understanding is that a smart contract is a protocol amongst people. You don't code it anywhere in the blockchain directly. It a a protocol that explains how, if each party runs code independently, assuming everyone as a criminal, still achieves its goal. For eg, multisig is a smart contract, which is a protocol where participants decide what code they'll run (signing) and the wallet layer that makes them meet is a separate layer altogether, not in the blockchain. Sometimes this layer may not even be required, for example in money-laundering or any of the 'hundis'.