I would say that this would be a judgement call. If as you say they are completely straddling the line, meaning that they voluntarily went out of bounds to stop the player it should be a violation per the new rule. However, I would think that if this same play occurs and the defender just happens to put their foot on the line then I probably wouldn't call the violation.

If others have case plays that differ my interpretation please post so I can change my thinking on this.