An important point to note for these rankings is that the are mostly about future value. Silver says in the third paragraph:
Two things we can’t stress enough: First, these ratings are purely statistical; there’s no manual tweaking involved. And second, focusing on the long term makes a lot of difference. Tim Duncan doesn’t appear in the top 53, but Jusuf Nurkic does.
I'd argue that LeBron is the most impactful player in the league, he most greatly increases a team's likelihood of winning, but he's ranked fourth here. Mainly because at nearly 31, he going to add less future value than 22 year old Anthony Davis.
Smart's rank is keyed to his age and the high likelihood that he will have many productive years ahead of him.