Please read the blog post!
On the plus side, it does somewhat explain the weird patterns in his diff image which I had been puzzling over.
The index_tiled.html version later in the article is what justifies the success claim IMO, and is the version I think it would've made more sense to host.
The currently hosted index.html just feels like a consequence of the author taking a scaled/compressed screenshot and asking Claude to produce an exact match.