It is very telling in the article itself there is a screenshot of the commits tab in the PR workflow that many don't realize even exists and/or never think to use.
In the Files tab the commit picker has gotten better in recent years, but it is still overly focused on selecting ranges of commits over individual ones, and there's no shortcuts to easily jump Next or Previous commit, you have to remember your place and interact with the full pulldown every time. Also, it's hard to read the full descriptions of commits in the Files view and I find I often have to interrupt my flow to open the commit in another browser tab or flip back and forth between the Commits tab and the Files tab in the PR. The Commits tab also defaults to hiding most of the commit descriptions, so it's still not a particularly great reading experience.
It feels like a bit of a bad feedback loop that GitHub's UI doesn't make commit-by-commit reviewing clean/easy because GitHub themselves don't expect most developers to write good commits, but a lot of developers don't write good commits today simply because GitHub's PR interface is bad at reviewing individual commits and developers don't see as much of a point in it if they aren't going to be reviewed in that way.