The study examined people blind since birth. But the article doesn't say much about the degree of their visual impairment—there's a big difference even between

The study examined people blind since birth. But the article doesn't say much about the degree of their visual impairment—there's a big difference even between "no light perception" and "light perception only" (where people can still get visual information useful for orientation), much less higher amounts of vision. If the study was restricted to people with no light perception since birth, that's a very small population, getting a larger sample might be difficult.