To be fair, the author notes this and offers an alternative:
> One immediate thought is that this is completely superseded by the regular browser scroll bar that’s ever-present at the side of the page. However, the scroll bar could be deceiving. If your page has a comments section, the comments could make the page look dauntingly long. Similarly, references to other pages and general “footer material” count towards the scroll bar, but would not count towards the progress bar.
> Combining the two, you could imagine an always-visible table of contents that highlights the current section you’re in. With such a feature, you can always see where you are (including a rough estimate of how far into the page you’ve scrolled), and at the same time see how the current section integrates into the broader structure.