Basically reference counting requires grabbing a number from memory in one step, then increasing or decreasing it and storing it in a second step.

This is two operations and in-between the two -- if and only if the respective memory location is shared between multiple cores or caches -- some form of synchronization must occur (like locking a bank account so you can't double draft on two ATMs simultaneously).

Now the way this is implemented varies a bit.

Apple controls most of the hardware ecosystem, programming languages, binary interface, and so on meaning there is opportunity for them to either implement or supplement ARM synchronization or atomicity primitives with their own optimizations.

There is nothing really preventing Intel from improving here as well -- it is just a easier on ARM because the ISA has different assumptions baked in, and Apple controls everything up the stream, such as the compiler implementations.