I disagree with that whole post.
No, shooting few threes just means you're not a focus in the offense. While sample size can be an issue, it can't ever be used to confirm the absence of something.
On the Duncan attempt at a strawman, yes, he could have learned to shoot threes if the Spurs had wanted him to. He had three-point range in college, and he feels comfortable taking a shot out to about 21 feet now. I bet he'd be a fine corner-three shooter next year if he worked at it this off-season.
I shouldn't have to address the fallacy behind using sample size as an argument for negation again. I will say that Jefferson's mechanics were wrong, which is why it took him a couple of years to become the shooter he was in his final year and change with the Spurs. He even admitted as much.
In short, there have been numerous examples of players learning how to shoot three-pointers late in their careers. It's really not hard to learn how to make a wide-open shot from a particular spot on the court, and that's pretty much all he has to do in the Spurs' offense. Not being a great shooter didn't keep Leonard off the floor early last season, and if wouldn't prevent Mbah a Moute from at least getting in for small-ball lineups. If the Spurs can deal with Duncan and Splitter on the court at the same time, they can deal with Mbah a Moute.