The real Alfonso was crowned king after the death of his father (so he was no longer a mere prince during the French invasion) and he was never captured by the French. He fled to Sicily, where he abdicated on his own son, Ferrante II, and became a monk.
King Charles uses a choke pear to torture Alfonso of Naples. Choke pears were unknown before the 17th century, more than 100 years after the shows' time frame.
The French would not call Alfonso a prince and his father Ferrante a king because they considered them nothing but usurpers. Charles VIII invaded Naples because he thought he was the rightful and only Neapolitan king.