This is probably true in the sense that it's also probably true that people are, at the margins, successfully manipulating votes, but it's part of the premise of the site that the moderators are working to counter that. And, like vote rigging, it's a cat-and-mouse game, so you're never going to get a wholly satisfying explanation of how they've decisively resolved the problem. Like a lot of problems in security and antiabuse, the practical solutions are essentially economic in nature: defenders do enough work to make attacks too expensive to be worth it.