That's the tricky part for these types of players deciding to go to the draft.
I think the rules are a little different for big men. A big will get drafted on potential much more often than a guard. Diallo's freak athleticism puts him in "draft on potential" watch, but I wouldn't necessarily call him a game-set-match lock to be drafted if he doesn't perform, especially considering the NBA isn't in desperate need for shooting guards at all right now, especially one that isn't a lights out shooter. Check the results from the last 4 or 5 drafts. There are some picks high in the first round, but most are great shooters or have some other unique quality. Someone like Wiggins is a good 2 inches taller than Diallo with similar athleticism. The NBA wants shooting guards at 6'7+, and shooters, these days ideally. Obviously Diallo is unique in his athleticism and transition ability, but he's still quite thin and MUST become a better shooter to make it at the next level. We've moved away from the shooting guard heavy offenses of the 90s and early 2000s. I hate to say it, because I loved that style of play.