This is a bit clickbaity. Sure, technically no one knows how yet. But we do in general know how these hacks happen - someone got the private keys. Given that it's the MyCrypto founder that discovered this... it's probably a leak from MyCrypto. Whether it was poor security, some phishing attempt followed by key loggers etc. or just a plain old disgruntled employee stealing the keys, we kind of know how this happened. Good thing that people storing that kind of money spread their cash around and ensured it was all in FDIC insured banks though...