Actually the confidence interval test is valid with almost any sample size ...
This is incorrect. Sample size affects the confidence interval test.
Going back to gambling, rolling a hard 10 hop is a one in 36 chance.
And yet, it is not hard to sit at a craps table and roll the dice 400 times and only see a single hard 10. It's just too small a sample size.
In modeling statistics of purely random events (e.g. a dice roll) it requires a very large sample size for any deviation to have any real value.
Your post tile claims "Reverse Engieering is not 20%" That conclusion is WAY too broad for the data you present.