these aren’t the actual maps from iNaturalist, but they use the observation tiles from the iNaturalist API. you can see that at the same zoom level, the gray circles almost completely cover up borders and names, while the red squares allow some of that detail to show through:
i believe that was what was tried initially, but there seemed to be a common concern that making the squares too transparent made the isolated low-density ones hard to spot on the map. so the product released to the masses uses a higher minimum opacity for the red squares. see the discussion here: https://forum.inaturalist.org/t/open-test-of-map-tile-improvements/7833.