Sorry, I just had to post this.
The diagram you're looking at is a standard tool software engineers use called a UML class diagram, and it lets developers effectively communicate their ideas about code layout. If we had to make a new UML class diagram for Mixxx in 2010, it'd probably cover one of the walls of my apartment. Although the MixxxApp, ControlObject, EngineObject, and SoundSource classes still exist (!) and might even be recognizable from their original incarnations, pretty much everything else has changed drastically.
The Mixxx architecture has expanded to accommodate an arbitrary number of sound devices, MIDI controllers, and soon, and arbitrary number of decks. Good things are in store.