To answer your question, I do not think that curl definition will increase with length if there is no curl to begin with. If one's hair is 2 inches or 24 inches, if there is no curl then there will be no curl (am I making sense lol?)
To your point about people learning to work with their hair to increase curl definition, what may be happening is that they learned to better moisturize their hair so instead of seeing frizz you see their curl pattern. Or they could be wearing a twist out or braid out
. Hope this helps.