I don't see what's so hard to understand about that statement.
If you are the best hitter in baseball, you're the most productive. If you aren't the most productive, you aren't the best.
We could argue about how to define "most productive," my way might be different than yours. But whoever the most productive is, is the best.
In my opinion, Harper was the most productive hitter in 2015. We know that power and on-base skills create the most runs for the team. Harper's power and on-base skills were well beyond those of any other qualified player, making him the most productive. Which makes him the best hitter in 2015.