I can't help but feel that you didn't fully understand what the grandparent post said, so let me take a whack at it:

First, human trafficking is illegal, but (apparently) very profitable. It was an unstated assumption that those people who engage in these sorts of activities do so primarily out of a motive for profit. In other words, that human trafficking could be largely eliminated by making it unprofitable.

In order for traffickers to enjoy the considerable fruits of their (slaves') labors, they need to invent a plausible-seeming and, more importantly, legal source for their wealth. This is called money-laundering. To do otherwise would invite the attention of the local constabulary.

What the GP is suggesting is that a viable means to curtailing human trafficking is to make money laundering difficult. This makes it difficult to use the profits of the crime, which reduces the incentive to commit the crime in the first place.