A lot of these guys make the same mistakes by drawing too much attention to themselves, when they start flashing the proceeds of their crime. He also kept the stolen stash close to home, so they just raided his property and found all the coins that he stole.... so all the evidence are centered in one place. 
There was no way he could lounder a huge amount like $3 billions, especially when he's autistic with no social calibration. Per article, he cashed out $700k just to have a pile of cash "like in movies". He was doomed to fail.
Funnily enough, he was involved in Bitcoin since early days, so if he just bought some (with the money he was spending on coke and booze) and hold, he could be rich and his wealth would be legit.
Anyhow, since this story is pretty interesting and Press board is not really meant for discussions, here's the topic on the main board: https://bitcointalksearch.org/topic/geeky-crypto-thief-32-jailed-for-hiding-3billion-in-bitcoin-in-popcorn-tin-5449502
