If he send 1BTC to 1400 owned addresses, though, no one can prove that the person that owns those 1400 addresses is the hacker, right?
Lets say there is the following chain of transactions:
1,400 BTC from a known hacked wallet to a known hacker's address
That 1,400 BTC is then split in to 1 BTC chunks and sent to 1,400 other addresses
Someone then deposits 1 BTC from one of those addresses to an exchange
What is the chance that the hacker has sold all 1,400 BTC simultaneously in exactly equal 1 BTC chunks and sent them to 1,400 different addresses? How did he sell them all so quickly? How did he find enough buyers? Why did every buyer want exactly 1 BTC? The chances of this happening are astronomically small. The only logical conclusion is that the same person depositing that 1 BTC is the same person who hacked the 1,400 BTC. Even if the hacker bounces the 1,400 BTC around multiple different addresses in multiple different amounts over multiple different transactions, it is still fairly trivial to trace them all back to the initial address.
That's why you use mixers. Good mixers will completely break the chain between inputs and outputs.