It's an interesting question. The referees reasoning was that the ball was kicked straight out on the full, so the restart would be where the ball was kicked from, and that's where he said to the TMO he was going to give the kick, on the 15m line.
But when the play restarted the kick was on the 15m; in line with where the incident took place.
Under Law 10, misconduct while the ball was out of play, the PK would have been where any restart would have been (back where it was kicked from), but as the whistle hadn't gone when contact was made, then the ball was still in play. So we are back to law 21.
I would love to know on what basis they have appealed?