If I'm understanding the article correctly, as long as the guesser can't see the coin, and chooses randomly, it is a fair 50/50. I don't think you even need to

If I'm understanding the article correctly, as long as the guesser can't see the coin, and chooses randomly, it is a fair 50/50. I don't think you even need to flip the coin. The coin could just be sitting on a table in the next room. If the guesser is ignorant and chooses randomly I don't think you even need the coin. You could just write heads or tails on a piece of paper, cover it, and let them guess.