Yeah I realize you can't just extrapolate the 40 time to the 100m result.
In this article Dennis Green is talking about how Moss acceleration is so sudden, that he will get to top speed quickly, which is why they like to use him on crossing routes over the middle of the field.
When a receiver is running a route once they beat the defender they will try to stack the defender behind them, by not running at full speed, then burst at the last moment as the ball arives. Moss was excellent at doing this with his size jumping ability and arms to just beat you that way.
I recall a game where Moss was matched against Darrell Green, and even though Green was near the end of his career he could run with Moss in his prime.