That is true and the FBI is requiring running in RAM (i.e. software) with no changes to firmware. Lack of SE appears to make that possible but I'm not 100% sure about that. My argument against doing has no basis in the difficulty of doing it. Although I would say adding a brand new interface to accept passcodes and test them is nontrivial.
That argument is simply that Apple has made an unbreakable system (and will with one more change) and should not be forced to break it by building a general purpose back door. Apple is merely building the inevitable unbreakable system for data at rest. The inevitable system for data in motion (comms encryption) was built in the 90's despite government's insistence that the world would come to an end if end-to-end encryption were possible.
Not a whole lot of people still use PGP but there are apps now that do the same thing used by literally a billion people according to BBC. Are we going to tell a billion people, no, you need to go through the Clipper Chip (or a software equivalent)? No, that is stupid. Going against Apple will be seen (is seen by anyone who studies it) as just as stupid.
The FBI is not asking for any “backdoor” functionality or for weakening encryption. The changes they are asking for regard limitations on sign-on attempts and escalating delay intervals between sign-on attempts.