i don't know. your argument feel intuitive, but itistoday has concrete examples of why the situation was actually positive. that "A 9 year old is emotionally and mentally streets away from a teenager" is an abstract concern to me - what exactly is the problem here? compounded by the fact that things turned out fine for the kid in the story.
People frequently have positive adulthoods after suffering abuse earlier in life. That's mere correlation, not causation, and doesn't mean the abuse is good.
People with pleasant childhoods can turn out bad. People with get beaten every day as kids can turn out good. That's as much not an argument to beat children as yours is to give them free access to adult, sexual situations.
So are you arguing that the porn was unpleasant to him? Because if it wasn't unpleasant to him at the time (like physical abuse would be), nor did it hinder his development, then where is the harm?