It is as easy as finding a duplicate signature. And while it takes some computational power (pennies comparing to PoW), you need just one watcher to guarantee that chain will remain honest. So, 1 honest person out of n. Almost as good as it gets.

Whole spiel about "only EF will run those, whole thing is centralized" is just noise. You as an attacker can't distinguish between situation when I'm running a slasher from one when I'm not. Now go and risk the attack.