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I don't think the hack is the index as much as it is the visual skip-list of sorts, and then being able to jump to the page with the content without having to do a binary search of sorts for the page number you are looking for.

That said, physical journaling is a hard habit to keep :) I feel like I've tried every trick under the sun. Bullet Journal seemed to work for a while, http://www.bulletjournal.com/ [no affiliation], but it's still tough to rigorously keep an index up to date when you just want to jot down something quickly.

The article describes a novel system subtly but importantly different from a simple index.

I don't see any mention in that entire wiki page of marking the edges of pages vertically for reference when looking at the book from the side. In fact it flat out misses it in the examples, "pointers are typically page numbers, paragraph numbers or section numbers" all of which require you to flip through pages manually instead of just looking at the side and getting a visual.

This seems to be a very specific type of index; can you find early examples of the kind demonstrated in the article?