We're also talking about two different ISAs: x86-64 vs A64. In addition to the underlying architectures to implement them. It's not inconceivable that Intel s

We're also talking about two different ISAs: x86-64 vs A64. In addition to the underlying architectures to implement them. It's not inconceivable that Intel simply made enough arch / process / design mistakes that they've ended up at the end of a performance box canyon. It's happened before. Pentium 4 was a deadend, while Pentium M-derived architectures continued to improve performance. So "Apple realizing continued gains by optimizing a less mature architecture" is one hypothesis. But "Apple realizing continued gains by optimizing a more evolveable architecture" is a valid alternative.