@LegionZero
Can you prove that you need Hagoromo's chakra to use the SPSM? Can you prove that Naruto has that chakra in principle?
You probably don't get it, but Naruto is not a book, it's an illustrated comic book where text is juxtaposed with a picture. No need to claim Naruto is using Kurama's chakra in SPSM if we are visually shown that Naruto's eyes are vertical in this state. Kishimoto has overestimated the mental capacity of his readers, apparently.
He needs a giant chakra that surpasses even Kurama Mōdo, and he also needs the energy of nature that allows him to access this Senjutsu. The method Naruto uses to create Senjutsu is simply Sage Mōdo, which is why Naruto's eye takes a horizontal shape. The method that gives Naruto a huge amount of chakra is "Kurama Mōdo" which is why Naruto's eye takes a vertical shape. And this power is presented as individual to Naruto, separate from Hagoromo's power, according to the short story "Shikamaru Hiden" and the official Databook.