You said he was an average junior runner. That’s untrue. He was a world champion at 15, U20 record holder at 16, and Olympic finalist at 18. Even saying he was an elite junior runner would be underselling it.
You make a good point about 100m vs 200m, but that’s actually where I think his height plays a factor. He clearly had borderline-GOAT top speed at a young age, but his height makes starts out of the blocks technically challenging (it was always his biggest weakness). I don’t think him needing an extra year or two to refine that is out of the question.