From the coder's perspective it is. There's just different syntax for assigning to them.

No, from the coder's perspective it looks like I'm passing an argument into a function. I suppose you could say that arguments in general are "mutable references", and I guess that's not necessarily "untrue", but that's not generally how I visualize it.

If I pass a persistent structure into a tail call, it looks like I'm passing a copy into there, in the same way that I might if I did something like `Math.abs(myValue * 3)`; the compiler converts it to a mutable reference but I see it as the same as passing a value.