Recently a man was deceived by someone who had change the electrum source code on an old version of it. He lost 1400BTC(!)
This is a sad story but you never provide a link or another other form of evidence to be sure the incident wasnt a fabricated story.
Back to your the why mixers? Crypto mixer come in the picture when people are advised not to use a single wallet thrice for security reasons.
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? They can only ask them where did you get that one bitcoin.
Creating 1400 addresses would be stressful and it will be better to move those coins to privacy wallets while send some portion of it once in a while to the tumbler site.