While this may be true in pick-up games, among actual basketball players this might not be true.
Here is a study from Harvard Sports Analysis (a student run analytical group at Harvard) that shows there is a direct correlation between college FT % and NBA 3P%.
What factors matter most when predicting success at shooting threes in the NBA? The Harvard Sports Analysis Collective demonstrates that to evaluate shooting talent, free throws are essential.
harvardsportsanalysis.org
Now, we have to take this with a grain of salt because Jackson attempted 10 FTs last year, which is not at all an conclusive sample size. If he shoots 80%+ though this next year with a real sample size, then there is an argument he could develop a good outside shot in the future. Or, he could just develop a good outside this summer.