What good is open source, when the developer can still add a backdoor later and put the backdoored version on iOS store?
I still need to trust the developer.
(And Apple, too, but once I can't trust Apple I can no longer the OS itself and just throw the phone away)
But, yeah, in theory, you can download it and compile it. The fact that you would have to also buy a certificate from Apple to do it is just a detail. You _can_ do it.
You also have the baseband CPU on many devices, which can read unencrypted memory anyway. So it is a hopeless case. Which is why this whole secure text craze HN is on is just insane. You'll never beat the momentum of iMessage or WhatsApp. But worse, there is not even any point! Trust begins and ends with a closed device.
Most of us are not going to be a target of state-level intelligence agencies, which are the only possible actors that have the sophistication to compromise the baseband processor. The idea is to prevent dragnet surveillance.