In principle, it makes no sense to base your theories on the idea that the characters in the story are lying to you, and as dumb as the Nika stuff is, the Gorosei are familiar with the existence of the fruit, like they are the reason we know it exists. Obviously, part of the Gorosei´s narrative role is knowing things, and it would make absolutely no sense that they don´t know that there´s a race of people who can eat multiple devil fruits that has existed for 800 years.
