It's pretty valid I think. There's a difference between being a set-shooter and having a good jumper. Nelson had a nice 3-point %, but he really didn't take many jumpers in college. Nelson was a set-shooter in college. Either he stood still and shot wide open shots, or he drove all the way to the rim. I can't think of a single time he took a shot beyond 10 feet when coming off of a screen or taking a dribble. When people talk about jumpers (especially for smaller guards), they mean actual jumpers, both off the dribble and off screens. That is a definite weakness in Nelson's game.
I wouldn't necessarily read too much into point totals in essentially pickup games against a bunch of guys who aren't likely to get drafted. We know Nelson can score against college players. What we don't know is what he can do against NBA players. Nelson has been successful in college due to aggressiveness and strength. In the NBA, though, the size, speed, strength, and athleticism is light-years ahead of what he'll face in college and what he'll face in the pre-draft camp. The scouts come to look at specific things that will translate to the next level, not so much stats.
There's really no point in even bringing James up here. James was 6'8" 240lbs with otherwordly athleticism and phenomenal court vision and ballhandling skills. Nelson is, at best, 6'4" (and may be shorter), and he doesn't have even adequate ballhandling skills for a guard. Therefore, it's much more important to be able to shoot a jumper for Nelson than it is for James. The jumper is just one of several questions about Nelson - it was pretty much the ONLY question about James.