From the description it sounds like Farsight simply keeps chunks loaded after they leave your normal view distance.
Distant Horizons creates LODs, which are simplified versions of those chunks, in order to reduce the load on the client machine.
Distant Horizons also has proper multiplayer support, meaning you don't actually have to travel to all the chunks in order to load them. A server with DH installed will just send the LOD data while you're playing like normal.