I'm not sure why they're making a big deal about this, couldn't the original Rosetta do this too? QEMU has been doing this since (I think) even before the original Rosetta, they call it user mode emulation. You run as if it was a normal emulator but also trap syscalls and forward them to the native kernel instead of emulating a kernel too.
I'm more interested in how they're doing the AOT conversion and (presumably) patching the result to still be able to emulate for JITs. That'd be (comparatively) simple if it was just for things from the iOS and Mac App Stores since Apple has the IR for them still but they made it sound like it was more generic than that.