Naruto doesnt have to knead kyuubi chakra. What he does is mix it with his own and manipulate it. He is master at controlling the chakra.Core chakra? Absolutely not. Kurama hands over kneaded chakra to Naruto. His BM avatar isn't even made by him. You can see the scans I gave to Micheal on the previous page. Which begs the question; where does Naruto fit in all this anyway?
You think Sasuke Susano'o is made by him? His eyes create the being. Sasuke puts in work by stabilizing the otherwise flaring chakra. Its a testament to both of their mastery over their respectiive chakras.