The fbi contracted a third party to defeat the limit on passcode attempts so they could brute force the lock screen by having someone just sit there trying every possible code. My understanding is that it involved physical access to the device and the way that process is handled by the phone was reengineered shortly afterwards to prevent it from happening in the future.
That’s not a backdoor, that’s sawing the front door into pieces to avoid triggering the anti-tamper system on the hinges and lock.
If you want backdoor worries, look to the generations of apple chips with undocumented memory mapped io registers that were in development during that time period. But don’t think too hard about how arm chips are developed or why that got left in there or how. You may come to the undeniable conclusion that a natsec cutout is licensing slabs of arm feature silicon with backdoors built in.
This is not a defense of apple. Only a clarification that there wasn’t a backdoor found in the San bernidino phone, and that if you wanna be freaked out about back doors there’s better stuff to get crazy over.