As long as you’re taking, and using, the address of that variable, it’s almost guaranteed to be in memory. Even if it won’t, the compiler guarantees the output of the program will be equivalent to unoptimized code.

> arguments will not be passed via the stack

I’m not sure explaining nuances of various calling conventions, and how they differ across processors and OSes, is useful information in a document about C and targeted towards beginners.

You’re talking about things which are underneath C in the abstraction layer hierarchy. The abstraction has many layers, the lowest one being quantum physics. One has to stop somewhere, and this article decided to stop at C, as opposed to assembly.