I got the impression that the reason the author chose JavaScript was not necessarily with the intention that people would go full-bore FP with it, but more as a way of "meeting people where they are"; JavaScript is widespread and many people are already familiar with it, so it's more likely that they can get up and running with the examples and start exploring the concepts more quickly than with, say, "Learn You A Haskell For Great Good" (where, at the very least, a lot of people would need to download and install the compiler).
And, if the author is successful, then the chances that you could one day be able to write JavaScript code in this fashion and expect it to be maintainable would increase!
This is exactly my impression as well and my personal experience. I first felt like I really understood functional programming when I started using Ramda. It was a library that provided me functional tools in a language I was already familiar with and that allowed me to play around freely without friction.
And, if the author is successful, then the chances that you could one day be able to write JavaScript code in this fashion and expect it to be maintainable would increase!