Earlier quoted context omitted.
Mutability is distinct from variability. In Javascript only because it's a pretty widely known syntax: const f = (x) => { const y = x + 1; return y + 1; } y is an immutable variable. In f(3), y is 4, and in f(7), y is 8. I've only glanced at this Zen-C thing but I presume it's the same story.
"immutable variable" is an oxymoron. Just because Javascript did it does not mean every new language has to do it the same way.
In a function f(x), x is a variable because each time f is invoked, a different value can be provided for x. But that variable can be immutable within the body of the function. That’s what’s usually being referred to by “immutable variable”.
This terminology is used across many different languages, and has nothing to do with Javascript specifically. For example, it’s common to describe pure functional languages by saying something like “all variables are immutable” (https://wiki.haskell.org/A_brief_introduction_to_Haskell).