Reversibility could probably be added on top via smart contract if someone wanted to provide the service, and people trusted the third party.

But you can't make a trustless exchange on top of a custodial framework.

Base layer has to be trustless / permissionless; all manner of third parties can build all kind of things on top of that.

But reverse isn't true - if base layer controlled by small number of third parties; nothing built on top can claw back that control.