The language at an early development stage. So "How immutable should variables be?" is open to discussion. You might want to open an issue and propose this in the issues section: https://github.com/chaos-lang/chaos/issues
Reassignment is mutation. It's really not... on a spectrum of mutability. JavaScript const objects allowing mutation of internal data is vaguely on that spectrum, even though it's misleading and confusing. Literally reassigning a variable isn't even up for debate.
Everyone says strings in java are immutable, yet you can still re-assign to string variables. It really is up for debate.
Java strings are immutable. Java variables with a string type are not. It's really not up for debate. You can't mutate a string in Java (you can in Ruby), but you can mutate a string variable by reassigning it.