A metal case will block any unwanted RF emission. That said, the article states that the goal is to make it possible for users to still use their smartphones as

A metal case will block any unwanted RF emission. That said, the article states that the goal is to make it possible for users to still use their smartphones as camera, GPS, or whatever tool you need on it, while being safe. As someone who knows a bit about computer security I find it mindblowing that journalists would go into war zones and expect to be stealthy with their smartphones in their pockets but they apparently are willing to risk their lives for their iPhones, so the best we can do is offer tools to mitigate the risks.