Before running the code, make sure your CPU does support AVX2. Most 64-bit Intel and AMD CPUs should. ARM CPUs, like those found in newer Apple computers, Smartphones or Raspberry Pis, won't support AVX2.
That sentence seems silly. ARM assembly and x86-64 assembly are two different beasts. Running the x86-64 code using Rosetta 2 emulation seems counter-intuitive as well if you want to get the fastest possible result on that machine.
ARM CPUs have NEON. I cannot say if NEON has all equivalent commands, however, the threadstarter is talking about complete rewrite, not just running it as is.
NEON only has 128bit register. AVX has 256 bit register.
Yep, it will most likely require to reorganize the program, but by itself it doesn't necessary mean that the result will be slower.