Think about it. Ines has never shown this kind of behavior in this timeline. That’s why ALL the nobles were surprised by her change. Her brother, waiting lady, her MIL and practically everyone in Mendoza were surprised by her change. The manhwa doesn’t emphasize it enough but all of Mendoza was shocked by her change. So how then is the crown prince admitting Ines has always been like this?
It suggests he remembers the Ines of the timeline he married her. He knew that side of Ines. They courted for 10 years and were afterwards married for 10 years. Ines already said in the flashback to the time she hunted with her brother and Carcel and Oscar was visiting, that those years were the best years of her life. That means any life so far.
Bruh how does Oscar know that Ines has always been like that ?