I'm still reading the article, but it looks like there's a typo where it refers to:
Merge in the stack from top to bottom - in this case we're
going to merge Feature 3's PR, Feature 2's PR, and Feature
1's PR, in that order, using GitHub's UI.
But the "top to bottom" would be merging F1, then F2, then F3 (and you'd have to rebase & re-upstream each as you go).