These guys aren't jumping up and down team draft boards because they don't exist yet. It's just that the media draft gurus are frequently way off on their evaluations.
Which of course could mean that he is way overvalued in mock draft or way undervalued in mock drafts. They could even have it right.
But, I will say that QBs have a history of shooting up even the real NFL team draft boards simply because of the importance of the QB position. I don't know if Dart deserves to go high. I didn't see it in college, but it's hard to really know what he could do in a NFL offense since Kiffin's offense is very much so not an NFL style offense.
And then there is the fact that the success of all of these young QBs with good enough athletic skills is dependent on how smart they are and what kind of work they will put in at the NFL level. There is really no way fans can know that and even teams are more or less guessing. Although they do at least talk to these guys, their coaches, their families, and friends. So they have a better idea, but they miss all the time on that too.