https://www.apple.com/customer-letter/

This is a letter from Tim Cook himself in 2016, regarding this topic. I might be wrong, but remember those articles talking about this subject and the master key for the iPhone. I've not read this document yet (just found it when researching to refresh memories).

The government suggests this tool could only be used once, on one phone. But that’s simply not true. Once created, the technique could be used over and over again, on any number of devices. In the physical world, it would be the equivalent of a master key, capable of opening hundreds of millions of locks — from restaurants and banks to stores and homes. No reasonable person would find that acceptable.

So without having full understanding whats going on, it looks like Tim was talking about an equivalent effect of a master key, when using such a tool (the firmware?). Maybe there comes this misconception in the media?