Is this a solution they rolled on their own? Cause it's abysmal. I've seen Leaflet being used for gigapixel images and it's great, even if it seems unorthodox.
They're using OpenSeadragon hosted on a codepen.
https://openseadragon.github.io/
https://codepen.io/Kimtaro88/embed/JjgLyJg/ee2ebbe7041869032...
Leaflet is actually listed at [1], I believe it's totally orthodox :-)
[1] https://en.wikipedia.org/wiki/International_Image_Interopera...