I notice that the map uses OpenStreetMap (via Mapbox, it looks like) for its base data, but doesn't display the required attribution[0]. For fixing this, their Attribution Guidelines[1] are pretty informative. Mapbox also has some helpful docs[2], and may have some additional requirements. Thanks!
Edit: After a bit of digging, I'm a bit unimpressed: it looks like the OSM and Mapbox attributions are deliberately hidden? From your compiled index.css: