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

  element.addEventListener('submit', submit)

The second argument to `addEventListener` is a callback that takes the event as its argument. (`element` should be the form and the event should be `submit` for accessibility reasons.)

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. The example uses a primitive value (`1`) which in JS are passed by value, so of course whenever you compare 1 with 1 it'll return true regardless of which variable the 1 is stored in. Objects, (that `event`s are an example of) are passed by reference. So you get

  const x1 = {foo: 1}
  const x2 = {foo: 1}
  
  x1 == x1 // true
  x1 == x2 // false
  x1 == {foo: 1} // false

So a simple comparison between objects is enough to determine if they are the same instance. But it won't work to see if 2 different objects contain consist of the same property-values as each other.