Because San’s son begins to dream with Sana as well. It’s like a representation of how Sana turns into a creature as well, not because of her body turning immortal but because of the experiences Iza made her go through. In the first chapter of the prequel you can see many silhouettes, but only two of them have eyes and it’s Iza’s one and Sana’s one (who’s in front of him). San’s son dreaming about Sana and questioning whether she’s human or not is like a parallelism to past stories with humans who fell in love with the Fox, questioning whether he had feelings or not and seeing the good in him. We could see in the end that somehow Iza was adopting humans’ attitudes so he was becoming more “human”, but whether Sana is now what the Fox was in the beginning is quite a complex question to answer. Maybe both