Sure, he can. Was he not involved in the writing of the Last? That completely rewrote 15 years of development when it said Naruto's feelings were just part of his rivalry with Sasuke. It may be inconsistent, it may be crap, but as far as the story is concerned, it's canon. Boruto, which has his seal of approval, has done the same thing.
i learned that a romance writer mostly wrote the last movie not kishi; and the last chapter might not have been written by kishi. Not to mention the supposedly canon gaidens weren't written by kishi but he illustrated them and the anime is using filler to add to them so those aren't canon to me, not to mention the other plot holes