A sample size of 100 is fairly reasonable, isn't it? The worst-case standard deviation is 5%, and 100 is a small enough number that you can take the time to carefully inspect each account (by humans) to actually determine if it's a bot or not. So 100 is something like the sample size I'd expect them to use here. Not sure why that would need to be a secret.
(The key thing is that the sample has to be truly random, which is usually very hard to do, but Twitter can easily pick 100 random accounts in their own database.)
(The key thing is that the sample has to be truly random, which is usually very hard to do, but Twitter can easily pick 100 random accounts in their own database.)