They might be using something like an NX bit on the generated x86_64 page, so that whenever the code attempts to jump into it, a page fault is generated, and the kernel is able to handle that, kicking in the JIT compilation and translating the code / address. This is essentially a "double JIT" so there will likely be a performance hit.

Since they control the silicon, Apple might also be leveraging a specialized instruction / feature on the CPUs (e.g. a "foreign" bit that's able to mark memory pages as being from another architecture, and some addressing scheme that links them to one or more native pages behind the scenes)

Maybe the A series chips even have "extra" registers / program counters / interrupts that aid in accelerating this emulation process.