If I'm not mistaken, this isn't the full Y combinator yet. It's a partial step towards deriving the full Y combinator.

    (f => (x => x(x))(y => f(x => y(y)(x))))
    (func => n => (n === 0) ? 1 : (n * func(n - 1)))
    (19)

Notice that the ugly func(func)(n-1) recursive call with the proper Y combinator simply became func(n-1).

The full Y combinator is also vastly more mystical and less obvious than the partial step :)