The author is correct in saying that JS was not designed for functional programming. The issues they raise are real issues, and a different language design would make functional style much easier to write.

The issue with immutable data is not that Javascript's model is confusing, it's that there is no way to write "this array is immutable." Without that guarantee, you can't allow multiple data structures to reference the same copy of data because the data might get modified unexpectedly.

There is a similar issue with tail calls. The compiler could replace calls in tail position with jumps, but it's important that this be part of the language spec. Without a guarantee in the language spec, code that runs fine on one system may blow up with a stack overflow on another.

It's impressive that JS works as well as it does when written in a style different from what it was designed for. Many languages make other styles so difficult that it's not even worth trying (looking at you, Java).