Explain how Naruto was able to do Kage Bunshin before manga? Exactly...filler.
He couldn't, calling a comic relief as "success" is painful.
Also, we already discussed many times that what Naruto read and learned from the Scroll Of Kinjutsus was the TAJU Kage Bunshin and not the normal Kage Bunshin. The difference between them is like Amaterasu and Izanami-Izanagi.
Read the RAW version of the first chapter and you see that what Naruto read and learned from the scroll was Taju Kage Bunshin. Even in the anime it's Taju Kage Bunshin and not normal.
Another note: Mahiru from Iwagakure from the Kakashi Gaiden knew Kage Bunshin despite being an outsider so 100% he didn't got the scroll and since only Hokages should touch the scroll the chances for Kiba to learn Kage Bunshin from the scroll was zero. Conclusion? While the Taju Kage Bunshin is a kinjutsu the normal Kage Bunshin is not.
So, since unlike Taju Kage Bunshin the normal Kage Bunshin is not kinjutsu and every second character could use it in the manga (even Deidara, Mahiru, Kiba etc) it means Naruto has saw it before and tried to use it like - but failed and ended up as a comic relief. Have I remind you that Naruto did the same with Gudodamas whiches he never used in his entire life but sawing Obito and Madara using them he learned them just watching them?
So it is explained! No filler.