It's absolutely not clear that the US has backdoors into any Apple product. Apple has fought pretty hard to ensure that their devices remain something that a user can feel safe and secure storing their private data on.
But if we're talking about plausibility, then it's much more likely that your underlying operating system, regardless of vendor - Microsoft and Apple are the major players - has been compromised in some manner, or contains the hooks for on-demand compromise if compelled by a state actor.
But if we're talking about plausibility, then it's much more likely that your underlying operating system, regardless of vendor - Microsoft and Apple are the major players - has been compromised in some manner, or contains the hooks for on-demand compromise if compelled by a state actor.