https://www.youtube.com/watch?v=vTH-OheNf9k
The reason why these Africans couldn't have learned of Sirius B and gotten all of 20th century knowledge from Aliens from Sirius B turns out to be a quirk of astrophysics.
Sirius B is a white dwarf. White dwarfs are usually remnants from stars like the Sun. Small stars that don't go supernova within a hundred million years. Instead, it shedds outer layers, after the star lives for ten billion years on average. But, Sirius B is a few hundred thousand years old! How did that happen?
Well, the Sirius star system is a double star system. One star stripped the layers off of the other star, leaving a white dwarf!
The Sirius star system is only a few hundred thousand years old. That's not enough time for life to evolve to multicellular life, then evolve intelligence after that. Besides, the star system is probably still getting asteroid bombardment of the planets. And, usually, double star systems are too hot for Earth like planets to allow life to develop.
