It doesn't really matter if they can track your coins after they leave a KYC exchange or not. They still know how much you withdrew and will assume you have them in your possession when they decide they want to steal them or tax you. Under the threat of fines or imprisonment they can compel you to open your books to them to show where those coins went. That's the biggest threat of KYC.

Personally I think that by the time they get desperate enough to try to pull some shit like that they will be completely bankrupt and not have the resources to pull it off, so I'm not too worried about it.