Would you have preferred if I had written "Apple does not actually need a backdoor per se in order toto perform the actions mentioned in the article?" My point was that what law enforcement is asking does not require a backdoor, since a lot of posters seem to think it implies there must be one. Furthermore, security researchers can and do look and see how all the signing keys etc are structured on running systems even without source code access. Is there a chance there is still something hidden, sure, but there is also a chance someone snuck a root exploit into an innocuous looking commit in an important open source project. Source code access generally does lead to more trustworthy code, but it isn't so black and white as you claim. In the end we depend on people to validate what we use, and just having the source available is not in and of itself validation.
As for the rest of the your comments, you simply don't know what you are talking about, but you would if you had actually read the PDF I linked. First off, rewriting the bootloader via JTAG is not an option on a lot of SoC's and embedded devices once they have had some of their internal fuses blown. From the PDF:
"When an iOS device is turned on, its application processor immediately executes code from read-only memory known as the Boot ROM. This immutable code is laid down during chip fabrication, and is implicitly trusted. The Boot ROM code contains the Apple Root CA public key, which is used to verify that the Low-Level Bootloader (LLB) is signed by Apple before allowing it to load."
So the stuff in flash might be rewritable, but it won't be executed unless it is signed. Reading the raw flash is also completely useless, because all data written to it is AES encrypted via a DMA engine in the SoC that uses various different keys, but all of them are tied to or derived from values fused into the processor and not readable via software or JTAG (they are routed directly to the DMA block and never exposed). That means the brute force needs to be attempted on the SoC in that particular iPhone, or you need to drastically increase the search space. A suitably advanced attacker code probably also obtain the SoC keys by decapping the chip, dying it, and looking at the fuses with a scanning electron microscope, but I generally don't worry about an attacker with sorts of resources; they would probably just beat my PIN out of me...