Even though I think what Kishi is doing is horrible and disrespectful to the characters involved and the fans, I will let go of my personal preferences for the time being.
Kishi's assistant lied because he had to, otherwise the plot of Gaiden would be spoiled, the same goes with all of Kishi's post-ending interviews, content, and character descriptions. This is the malicious truth.
The theme of this Gaiden is bonds and relationships going beyond genetics. This chapter was dialogue heavy just to further establish that. Sakura immediately disagrees with what Shin says about parenting being about genetics and that that isn't what being a parent is about.
There is no point to all this dialogue, all this time wasted, if Sarada is not related to Karin. Furthermore, there is no lesson to be learned for Sarada if she just ends up being Sakura's birth child. At the end of this chapter, Sarada throws a fit over not being Sakura's biological daughter and doesn't even acknowledge her as her mother. Which is completely wrong because a mother goes beyond genetics. Who raises you is your parent. Which is the theme that has been established in this very chapter. Once again, if Sakura is the biological mother, then there is nothing to learn for Sarada. There is no life lesson, their lives would go back to as they were at the beginning of the Gaiden and there would be no point.
Now all I can hope for is for the way this all went down and the reason behind this is properly justified, and does not involve cheating, for Sakura, Sasuke, and Sarada's sake. My current guess is that Karin helped Sasuke and Sakura by being a surrogate of somekind because Sakura couldn't conceive.
