Yeah, it's strange - a lot of ES6/next tutorials seem to think that FP concepts in JS just showed up.
I don't get it.
Possibly says something about how long it took for people to trust ES5 was supported by the browsers they were writing for. It seemed to some people that ES5 and ES6/ES2015 were added to browsers "at the same time", due to legacy effects of browsers many felt a need to support for far too long in major enterprise environments (<cough>IE6</cough>).
I found it confusing it was mentioned in this article because of this. I wonder if they just wanted to show off how elegant arrow functions are and used those as an example?