Natsuo's spotty memory of his childhood isn't surprising. Childhood trauma and domestic violence in particular tends to cause memory problems especially concerning memories of those events.
What is also true to life is that he mostly recognizes the abuse through the treatment of others - particularly Rei and Shouto. Since Natsuo wasn't valued at home, he was able to recognize the wrongness of that environment through Rei's experience - and eventually Shouto's experience with abuse.
It's explained more in the light novels, but Natsuo disliked Shouto until the kettle incident. And that makes sense for multiple reasons. All children want some kind of connection with their caregivers. Natsuo like any child probably had a phase where he wanted to be close to both of his parents but was ignored. Considering how much attention Shouto received from both Enji and Rei and how close Natsuo was to Touya, it only makes sense that Natsuo would have initially resented Shouto.
The kettle incident clearly didn't bridge the gap between Natsuo and Shouto as demonstrated by how little Natsuo seemed to understand him during this conversation with Rei, but it changed Natsuo's perception of Shouto. His younger brother changed in Natsuo's mind from the reason their household was terrible to one of its primary victims like Rei (and Touya). One of Touya's biggest character flaws was that he didn't grow out of this initial mindset of viewing Shouto as an extension of his abuser.
