He's more than a year away from playing in a college game so there's plenty of time to improve the jumpshot. Plus, easily could have been an off shooting weekend. Doubt he shoots 19% from 3 all the time.
His form is fine, which is the first thing to look at. So as long as that's there, it becomes a question of 1) reps (does he work to improve?) and 2) shot selection.
I've got no doubt on question 1 - the kid is supposedly a worker. On question 2... My guess is it'll come with time. Jalen, for example, is a very good shooter, but didn't know a good shot from a bad one until late February. Last 10 games of our season he was over 40% from 3; first 25ish he was around 20%.
Chances are that on the AAU circuit a kid like Diallo isn't being coached to be selective. Whoever lands him - hopefully us - will probably have to do some work in that regard. The good news given Cash's improvement is that Ollie can both recognize and address the issue.