Did you read the article? The entire discussion was on the tension between immutable data structures and cognitive complexity on one side, and machine performance and mutability on the other side.
First, if I sounded snarky I apologize. I assure you it was not my intention.
Secondly, I did read the article. If they are allocating a big chunk of memory and copying it over every single time they are making a change, they are not using immutable data structures. Ok, technically speaking, they might be. But they are using the worst implementation possible as a base to make conclusions from. That'd be like looking at the C language and concluding that strong types are not good, because of all the limitations C has.
