The reason the FBI could get a third party to access the iPhone is because DRM, unlike real encryption such as what is used in Monero, is based on snake oil rather than sound mathematics.
Edit 2: The same or similar Apple DRM that the FBI broke, was used from 2009 to 2014 to censor Bitcoin and is currently used to censor Monero
Are you saying that Apple is lying when it says the user has the private keys and not Apple?
Probably not lying, but these systems are complex, always under development to implement the complex, possibly conflicting, and often shifting demands of DRM, and therefore very unlikely to actually ever be very secure.
Also, it is well known that intelligence agencies have infiltrated tech companies in the past and no reason to think this doesn't continue so there may well be back doors (facilitated by complexity in which they can hide) that "Apple" as an entity doesn't even know about even though someone working for Apple or an Apple vendor put it there.