When someone says Kurama isn't Naruto's, they mean that he wasn't born with it, nor did he work to achieve it. He didn't defeat it and seal it inside of himself, someone else did that, and it isn't a Kekkei Genkai. For a while now, Naruto has been completely reliant on something beyond himself. Naruto can't use KCM without Kurama giving it to him freely, therefore it's not his own power. Sasuke's Sharingan is his own power, by birthright of genetics, the EMS is simply a negation of blindness just like Sage Mode is a negation of the arm wound that Naruto got when he first used the Rasen Shuriken.