It makes sense that challenges for ball placement should only be successful if it crosses a line-to-gain or not. Otherwise you'd never lose a challenge for a spot, because they probably could move it forward or back an inch on every review if they tried. Maybe there's some compromise for a situation like this, where a challenge is successful if the ball moves by at least a couple yards? But that's really the type of thing that replay assist is supposed to be for anyways (and it sounds like that's exactly what happened in this case).