I think I agree with the article's assertion: function arguments should be immutable by default, unless explicitly marked otherwise. I believe this will prevent mistakes from functions modifying their inputs directly.

I don't think it's relevant if you consider arguments as variable assignment. Then just do whatever the rest of the language does with variables. If they're immutable, then they're immutable, if not, then you should be able to assign to them using whatever scope rules you have. See https://en.wikipedia.org/wiki/Evaluation_strategy#Call_by_value for an explanation of how languages like C and Scheme do it.