I am pretty much this was because of fat fingers, but how an earth there's no security measures for this kind of thing? It's 2020 already and after seeing this kind of mistakes, i am pretty sure that ETH adoption is further away. Just one warning message informing about the possible huge mistake could have saved millions for this guy.
I guess it's the wallet's task to check if a fee isn't outrageous, but i have no idear which popular wallets actually do a quick feecheck before broadcasting a transaction