True, but that is in the same sense that TSA can’t prevent air piracy.
The presences of security reduces the certainty that they will get away with it, and the uncertainty itself is a deterrent.
And it being the whistleblower herself raises red flags. They can silence somebody without such obvious red flags (and with plausible deniability as well) by killing their loved ones; often those who aren’t afraid to die for their cause are reluctant to have their loved ones killed because of them. And just like Glenn Beck found out, he can provide security for his family but not for everybody he knows and loves. Killing somebody close is a way to say that nobody associated with you is safe, because there’s no way to protect them all.
That’s why they cut wires/sheathing in the vehicle of a SPOUSE, for instance, in order to try to silence somebody. Or put a horse’s head in somebody’s bed... Same principle.