Not to discount the work or end result that went in to this, but when I saw Pixels I assumed it used a 'standard' library, e.g. Leaflet, used for many maps (which have similar zoom) and tiles. I'm wondering why you wrote something from scratch?
From the article: "For this project, I chose to use vanilla JavaScript with no external dependencies or libraries. In general, we strive to keep the dependency count and build process minimal for these projects, since keeping the complexity level low gives them a better chance of aging well in the archives."
Still, I think from a UX perspective the slightly springy behaviour of Seadragon is much nicer than the algorithmic rigidity of things like Leaflet or Google Maps.