Steph himself isn't responsible, and threes were definitely trending upwards, but you absolutely cannot tell me that he didn't hyper accelerate it after the 2015-2016 season. The impact he had on the game after that season was noticeable in every single park, every single basketball conversation, and really seemed to be a major shift and/or acceleration in the way that teams and players at all levels started approaching the game of basketball.
The reason why I think your comment is a little ignorant is that he's simply stating that when Steph had that insane 2015-2016 season Morant was 15. In my experience, that's when a lot of people really start thinking more analytically about the game. These guys who are early 20's were experiencing Curry madness in their early teen years and for sports I would argue that's when a lot of people get really into it for the first time while actually understanding on a higher level what they're seeing.