Well, the answer is – we don’t know. Because there’s very little information to be found about Dong Yi in the history books. All that’s known about her is that she came from the cheonin, servant class, then was noticed by the king, became a concubine, and eventually received the title Bin – the top rank for a royal concubine – when she gave birth to King Sukjong’s son. She was given the name Sukbin and her son became King Yeongjo who reined for 52 years and was the longest reigning king in Joseon history.
Dong Yi and King Sukjong blend in with the crowd, Dong Yi, MBC
One of the reasons there is so little information about her in the history books is that she came from the servant class. In the introduction to the novel Dong Yi, I read that in general, there is not very much information available about the royal concubines, as record keeping in the Joseon period focused on the king. But if the concubines came from the upper classes, the names and positions of their family members were more likely to be recorded. As Dong Yi came from the servant class, it would have been awkward to write anything about her, so information about her background was purposely avoided. That seems to be why there is less information available about her than other concubines. In fact, Dong Yi was not even her real name and there is no record of what her real name was; she is just known as Sukbin. (Choi was her real family name though.)
So, the drama is fiction, as nobody knows what really happened. Perhaps they really did have a romance. But some of the scenes are definitely unrealistic – for example, the scenes when King Sukjong (disguised as a yangban) and Dong Yi go outside the palace and spend time together drinking and eating at the hostelry. Because of the strict Confucian society rules about relationships between men and women at that time, they would probably not have been able to sit so casually together without attracting attention. But King Yeongjo, Dong Yi’s son, was known to go out of the palace secretly to find out how the ordinary people lived. So perhaps this is where the idea for those scenes in the drama came from. King Yeongjo (probably since his mother came from the servant class) changed some of the laws about marriage between the lower and upper classes which led to the servant class eventually dying out.