That was a fun read, but I think there's a third possibility that the author has missed.
Allen wasn't simply less accurate. If he were, then his passes would follow a sort of normal distribution, but his didn't seem to. Setting aside too-narrow windows and bad drops, his misses were often just wild pitches, far worse than simply a bad throw. Imagine a shooter who hits bullseye half the time and misses the paper a third of the time. If you can explain the instances where the shooter is missing the paper, then you have a coach-able situation. If it's just a shotgun type pattern then experience can improve the situation but only so much.
Allen had thousands of fewer reps than the big names and I suspect that the wild pitch/panic and bail instinct along with basic mechanics under pressure is an earlier problem to sort out with both coaching and experience than tweaking the position of your elbow or whatever. The former issues are rarely (if ever) a problem seen in big program college prospects. I think that is why old scout types understood the promise he had because they have been involved with actual players and have a more intensive understanding than the extensive view of the stats guys.
Incidentally, the "all the throws/runs" video on Jackson was similar. Dimes and wooden nickles. Jackson's field vision and placement were so often crazy good or completely off.
(looks like buffalo junction beat me to this, but I'm invested now.)