If you want to see it you can quickly get it (at least with the latest Mac OS, I don't know about Windows).
Having a thing across the top is perpendicular and doesn't relate to the vertical structure of a scrolling page. You have change the vertical position of your eyes to see it in a context where vertical position is significant.
Using a horizontal bar to denote vertical position is a tad unintuitive. I feel like this doesn't really solve the problem you are talking about, and instead adds to the confusion of readers who notice the bar.
Edit: Keep in mind that as I'm reading the story, my eyes are nowhere near the top of the browser. If you want to give me context, the top of the page is not the best place to provide it. It does make for a cool demo though.
Actually I read the article but never noticed that scroll locator at the top. Anyways it is kind of useless if you are on windows where the scroll bar is always in the view. Anyways nice article!
I was actually going to also have it display a toast-like "7 minutes [of reading] left" on the top left when you scroll a good amount.