Either way, unrelated to the point in the article about the advantage of immutable values for repl, not immutable bindings.

I would certainly agree that truly immutable bindings would be at best quite a stumbling block for repls, but so far no one was advocating for them as a good repl feature.

"Changes at any arbitrary time, including when you are halfway through reading it" is not in any way a sound synchronization strategy and the only thing you get with assignment.

You seem to understand the important difference between identity (what you see when you read an object) and reference (what you use to access the object). Next important thing on the list is how "reference" cannot just be a pointer to a place in memory -- unless it is immutable.

So def on an existing binding does assignment. That's what is meant by "re-binding".