I was looking at the sourcemap and it appears that they wrote their own tiling library (see Tiles.ts and related files under js/). Is this right? Why not just L

I was looking at the sourcemap and it appears that they wrote their own tiling library (see Tiles.ts and related files under js/). Is this right? Why not just Leaflet since they're rendering PNG tiles? Or Mapbox? The whole thing feels a bit over-engineered to me at first glance.