This is a prequel. Before the boy became the swamp, he made hundreds if not thousands of visits that ended just like this one. Eventually the swamp got enough of a taste of him that it latched on and the boy finally got what he wanted.

He just kept injecting his thought energy about wishing he could somehow really have sex with the mud. Somehow, the way our bodies will our neurons to fire and our bodies to move, ripples out into the world. Our thoughts shape these ripples. Our consciousness injects our imprint on reality-- perhaps that's how ghost are seen, one brain picking up another's thoughts/ imprint. But, here, Duke's thoughts were only, "I wish the swamp would gloop up into a person; just a giant ass to smash even."

We know, though, what ended up happening. The boy got so much more than his neanderthal brain could imagine.