They always can. Microcode updates come to mind. While apple have the ability to keep their ecosystems locked, they also have the capability to unlock them.
Probably not easy and not on mass scale. It may even need to have to see inside the silicone itself. Which is expensive and hard. But as long as the keys are on the device or in possession of apple - they can be extracted.
Worse case, you disassemble the die and read the bits off with an electron microscope. It's still possible, just expensive, painful, and maybe dangerous if you damage the chip.
Probably not easy and not on mass scale. It may even need to have to see inside the silicone itself. Which is expensive and hard. But as long as the keys are on the device or in possession of apple - they can be extracted.