Hot vs Cold wallets are a social construct. People talk about wallets being "online" or "offline" but if the keys are compromised there's no such thing.

I know, but if something is split up and stored in a vault somewhere the likely hood of a vulnerability napping the wallet falls. Defence in depth and risk analysis.

I don't know much about bitcoin but surely you don't store this all in one wallet? You split it into thousands and then use hardware security modules with a risk management system so it doesn't let you transfer $5bn instantly. Unless it was an inside job maybe?

I'd be really curious to know what their security architecture was.