In case there are people confusing what is happening in this scene, I will explain the situation.
That young image of Hinata and Naruto was not showing a flashback. They were coming through a genjutsu world where their memories were displayed as bubbles around them. They were also able to interact with each others dreams, as shown earlier when they were unconscious and Naruto saw how Hinata really felt about him. Because they were in the illusion, they entered each others memories at differing ages, and held hands at each different age shown. This is why there is a scene of them as kids, a scene of them as genin, a scene of them from after the two year shippuden time skip, and last as them at their current age. It was meant to be Disney-esqu romantic. The difference is Japanese sense for romance is a little different than other countries, and they think "cute" is romantic.
Either way, the only retcon that can be considered is the fact of what happened after they got out of the illusions. This is where the tooth rotting sugar came. Naruto bridal hugged Hinata, told her not to let go, cute stuff cute stuff, and he forms a rasengan with one hand and...shoots a beam out of it to propel them out of the water...Yes. One must take the good with the bad.