They probably made the mistake and later realized and decided to keep the natural order of play. In the end it was just one game they swapped server. And they lost the first 2 times Bea served (including the one she shouldn't have served) , so, I highly doubt they were getting any advantage from that.
Anyway, it was a great game, the opponents challenged them, and they scored 6-0 in the second set and managed a fantastic come-back in the 3rd set.
If all you got from the game is that they "cheated", I'm sorry for you.