I don't think that there's yet enough information to draw this conclusion. If it's the author's engagement that caused the spike in sales, what happens months

I don't think that there's yet enough information to draw this conclusion. If it's the author's engagement that caused the spike in sales, what happens months down the road when the author's post is forgotten, but the pirated bits are still floating around in torrents? While the effect looks positive today, it's entirely possible that the long-term result is negative.