Superman's explicitly non-magical powers (he actually has a problem with magic) are a result of his alien physiology, manifested due to Earth's yellow sun. His genre calls him out as not human, and his abilities come from his being not human. There is no outside genre conceits here.
And all Kryptonians have powers like Superman's because its a species trait of Kryptonians. Do all humans in the Forgotten Realms have magic Elminster? Did Elminister get magic because of who he is from a species perspective? Of course not, because your example makes no sense.