Heres the thing, and I looked this up because the ex ref on the show said this... It's not as soon as the slide begins. It's when they start a slide to declare down and anything besides hands or feet first touch the ground. I believe it was Josh's knee that first touched the ground and the ball was obviously past the mark by then. Plenty of video to see that.
Rule from the actual book copy/paste...
sliding or diving. When a runner slides or dives feet or head first or simulates sliding or diving, the ball is dead the instant he touches the ground with anything other than his hands or his feet, or begins to simulate touching the ground;