The Thorium system is another component of Salt with the ability to watch the event bus and react based on what it sees there. But the ideas behind it are much different than with the reactor.
Thorium is one of the engines that started shipping with Salt in version 2016.3. Engines are a type of long-running process that can be written to work with the master or minion. Like other module types, they have access to the Salt configuration and certain Salt subsystems.
Engines are separate processes that are managed by Salt. The event reactor runs inside the Salt processes themselves, which means that long-running reactor operations can affect the rest of Salt. Because Thorium is an engine, it does not suffer from this limitation.