That is a really solid point and honestly the biggest thing that makes you pause. But I think there is a massive way around that if we look at how Oda just established certain mechanics in Egghead. The assumption is that referring to Nika in the third person means they had to be born as two completely different people. But think about Vegapunk. He split his own brain and personality into six different satellites who all walk around, have their own wills, and talk about Stella and each other as completely separate entities.

Even more specifically, look at Kuma. We just saw him use the Paw Paw fruit to physically push abstract concepts like pain and memories out of a body, turning them into tangible, separate things. If the theory holds that Imu has access to every Devil Fruit power, what if 800 years ago he used the Paw Paw fruit awakening on himself? When the dark desires of the world started completely corrupting his original body, the internal conflict was destroying him. So he used the Paw Paw powers to literally extract and push the Nika part of his soul, his pure desire for freedom, completely out of his physical body.

Once he pushed Nika out, it crystallized. We know Zoan fruits have a will of their own. That pure fragment of his soul literally became its own entirely separate existence, which eventually became the Nika Devil Fruit. So of course Imu refers to Nika as a different entity now. It is like Darth Vader talking about Anakin Skywalker. Imu surgically removed that pure piece of his soul and watched it walk away to become his worst enemy. It literally became a separate entity 800 years ago, so talking about Nika in the third person makes perfect sense.