Additionally, that letter is written with very carefully fuzzy language so as to remain true-ish.
As in, Apple can't make this change and guarantee that nobody else will use it. In the same way that any software change (which they do regularly) could open up a security flaw. That doesn't mean it's likely, or any more likely than any other software update.
I wouldn't put it past the NSA to have snapped up a former iOS developer for explicitly the purpose of breaking into iPhones. An ex employee could certainly call him out.
> Additionally, that letter is written with very carefully fuzzy language so as to remain true-ish.
Cook is trying to be as clear as possible to a public who doesn't understand encryption or why Apple is standing up to the FBI. That's a lot to swallow, and yes his letter is carefully worded for good reason. Given that there is a backdoor that Apple could create, it's not unthinkable that with more details, 3rd parties could figure out how to get in too.
This sounds like a misunderstanding. No clever third parties are going to find a way to break code signing. To exploit the security hole Apple already created would require having Apple's signing certificate so that a phone could install the code. If it wasn't for the certificate, it would be quite a bit more trivial to hack a firmware to do a malicious thing and install it on any phone.