Right, and it doesn't even require actual tumblers. Any service where you can deposit and then withdraw bitcoin can be used as a tumbler: gambling sites, exchanges, bitcoin marketplaces etc. Each time this is done the link is broken, and to de-tumble you need transaction history from each of those services. It's like if you want to deanonymize someone who connects to Internet though 10 VPNs: you need logs for ALL 10 VPNs to deanonymize the user. Criminals can also exchange bitcoin to Monero through anonymous exchanges, let it sit for a while and then exchange back over time.