I'm curious why the attacker converted his ETH into BTC. Why not Monero first? Once the money is converted into Monero, it should be untraceable, right?
Liquidity. There's enough demand for ETH-BTC that you can convert $30m worth pretty fast; for ETH straight to Monero, you would be in for a much longer wait.
Monero is also not quite untraceable with the defaults. It requires understanding Monero a bit, and spending days churning your coins. The idea that Monero is so unlinkable is horrible marketing and is going to get someone busted.