You have no idea if this is true or not, he played against high school competition. Dante Exum looked like a superstar in high school. Then he got to the NBA and nothing worked. I would have been fine with Simons, but I also realize no one had any idea what he was, it's a bet mostly on physical measurables.
This is part of the scouting process. You look at data and what the player can and can't do and make projections. Allen is going to play hard, bust his *** on defense, make some open shots and be a rotation guard who can play on the ball off the bench. DL made it sound like he's going to be a role player. He may be good in this role, but he doesn't have the talent level to be a lead guard or a primary scorer.
Exum was a guy who was very raw and needed to develop physically and develop his shooting. He got derailed with the ACL before his 2nd season. Otherwise, he was showing real progress in his second summer league.
Simons already has a number of advanced skills to go along with elite athleticism. His ceiling is very high, but I don't think the Jazz want to wait on developing him. They want to maximize wins right now.