And while they were on the road a rumor came to David, saying, “Absalom has slain all the king’s sons, and there is not one of them left!” Bible

“a rumor came to David.” The Bible does not say how the news of what Absalom did reached David at Jerusalem before the king’s sons did, who were riding on mules. It is possible that someone at the feast had a horse and outran the mules, and if that is the case, it is also possible that the person left the feast immediately upon seeing Amnon in the process of being killed and rode away assuming that all the king's sons would be killed the same way, and thus inadvertently started the rumor.

Commentary for: 2 Samuel 13:30