Unfortunately majority of big breaches like this are a result of social hacking rather than some computer science magic. However to answer your question of how

Unfortunately majority of big breaches like this are a result of social hacking rather than some computer science magic. However to answer your question of how much you'd actually need to know? Decent networking and system understanding as well as how to apply this knowledge in reverse engineering. Finally you need loads of luck. Most of penetration testing is just throwing existing things at the system and generally looking around for flaws and if you're lucky you might just stumble on something valuable.