Comment by MertsA

I think the charitable explanation here is that this was an undocumented debugging interface. Apple knew about it and did not disclose it in any publicly available material. The NSA almost certainly has access to Apple's source code and documentation. Just look at the Snowden leaks when it was disclosed that the NSA was mitming Google's DC to DC links. They already knew Google wasn't encrypting those links before they surreptitiously dug up the fiber and they already knew enough about the system architecture to make sense of that firehose of data. Clearly either through NSL or bribing some insiders, they already exfiltrated a bunch of internal documentation and source code. Why would Apple be any different?

I wouldn't expect them to have HSM keys or anything but a mirror of their VCS? Yeah the NSA probably has that.