Almost never look at a kids percentage in AAU as a barometer for how they shoot. AAU just isn't structured enough to get a read on shooting unless they are just a lights out shooter
Partially true, but NBA scouts do look at it, usually in conjunction with FT%. They're concerned more with the player at 22 than 18, so form and potential to hone the shot are important, but if the player struggles on 3's and FT's, the player is never going to project as much of a shooter.
Kawhi Leonard was kind of famously (now) a bad 3 pt shooter in college (25%), but he wasn't a terrible FT shooter (74.4%). There are a lot of cases of players who couldn't hit 3's worth a crap in college who developed in the pros, but it's rare if the guy was a HS and college dud from 3 and the FT line.
Conversely, though, the truly elite shooters almost always show it at a young age. You don't "develop" a Steph, Klay, or Durant in the pros, they were born with something they developed themselves.
One thing of note is that the EYBL went with a 24 second clock and the international 3 point line this past year. They put these kids in a close to the NBA rule structure, so it's no surprise you saw some ugly 3 point shooting numbers.