Rounding Up
If you multiplied the world’s population by two, you still wouldn’t get a figure anywhere near 20 billion, and that’s even after you round it up to the nearest billion. Right now, there are approximately 7 billion people on the planet, so we have no idea where the 13 billion other people come from, according to this person’s post.
However, they did do an excellent job at standing out. And by that, we mean it in the most offensive, condescending way. Did this person count all 20 billion people in the world? No, just as the commenter didn’t. The commenter’s estimation was actually much closer to the truth, OP.