It is a typical language-comparison strawman. He is writing

   list.reduce((x,y)=>Math.max(x,y)) 

To get the max number in a list. And he is "winching" because he can't write:

    list.reduce(Math.max)

Because the variable arity of max does not play well with reduce. But because of the variable arity he can write:

    Math.max(...list)

Which is even simpler.

So he deliberately writes overly complex JavaScript code to show how the other language is more concise.