Someone had been working on this for a while, and had a specific combination of technical skills and attention to detail to make this happen.
I think the dataset they're using is https://archive.openrs2.org/ . It looks like NPC data might be a part of that. This version has the wrong NPC mappings, and also missing object data: https://osrs.world/?cx=3262.58&cy=8.22&cz=3265.93&p=-244&y=1...
Older Runescape 2 (pre-OSRS) caches have objects but no NPCs.
https://oldschool.runescape.com/
Does anyone know if old accounts would still work? From around 2000 ish.
To explain in more detail - OSRS was a 2013 re-release of a 2007 backup of the game, after community backlash against a series of unpopular changes. It has then evolved alongside the original game, with its own dev team.
Surprising, is it not just using leaflet or something similar for displaying the tiles? Or is it some 3d visualization?