It's funny how machine code is a high level language nowadays, for this example the CPU recognizes the zeroing pattern and does something quite a bit different.
It has it's merits, but the underlying hardware has changed.
Intel tried to push this responsibility to the compiler with Itanium but that failed catastrophicically, so we're back to the CPU pretending it's 1985.
> And, having done that it removes the operation from the execution queue - that is the xor takes zero execution cycles!1 It’s essentially optimised out by the CPU
But it will not execute xor, nor will it actually zero out eax in most cases.
It'll do something similar to constant propagation with the information that whenever xor eax, eax occurs; all uses of eax go through a simpler execution path until eax is overwritten.
Right, it has the same consequence, but it doesn't actually perform the stated operation. ASM is just a now just a high level language that tells the computer to "please give me the same state that a PDP-11-like computer would give me upon executing these instructions."
It's emulating the zero result when it recognizes this pattern, usually by playing clever tricks with virtual registers.