Re:Answers to your questions about Apple and security
by icebraining on Monday February 22, 2016 @11:50AM
(#11152171)
I think it's meant taking the key used to signed that custom version of iOS. That is, if the FBI can force Apple to write the custom version, they can also coerce the key from them to sign their own copies, preventing your suggested blocking mechanism.
This is what I understood, at least, from the parent's post.
Clearly it is not, otherwise they would have done this. They did not.