So perhaps it's a prototype, or somehow due to api limits, but why can't they add all tweets to their results? Number of followers provides a natural ranking mechanism.
I think you put a little too much weight in follower count. There are a lot of methods to drive up your follower count (following big names, finding accounts that auto follow back, etc). If your followers aren't people that actually chose to follow you because you provide some value, what good is your number in determining importance?
Follower count is the only mechanism that Twitter provides for determining worth -- misguided or not. With some sort of tweet-ranking (everyone's doing it these days), perhaps there would be a better way to determine "prominence".
-m