Basically, it is because they used sampling, so they drew a sample using the probability distribution given by softmax, which can technically make you draw any character in the vocabulary if its probability is non-zero. As they said in the video, they got "lucky" and drew a character that was matching the one they were expecting, and did it so the illustration would make sense. If it was not the sampling method, it would have been the argmax probability, and in that case you always pick the character that has the highest probability in the distribution (which is o in the illustration).