How so? When I consider how to build it on top of an existing smart contact platform you'd have to have a whole extra, nonstandard layer for key rotation and you'd miss out on privacy guarentees. If we assume their goal of privacy and sub 10s finality I can't see how they could have done it on an existing cryptocurrency.

What they've done is essentially take Monero as a base, remove mining and use Stellar Consensus then solve any new privacy issues that arise.