> To avoid the whole "what is the `event` variable and where does it come from?" problem they could use

Why avoid something when you can instead learn how it works? :)

> Also, the author's example comparing `x === window.x` as evidence that it's insufficient to know if the in-scope variable is the same as the one on the window is wrong.

I think you misunderstood it.

When you pass an object as an argument to a function it's passed as a reference. It would have been clearer if I used an object (I was trying to simplify the example) but my code is still illustrating what I meant it to even if I substituted `window.x = {a:2}` for `window.x = 1`.