This isn't 50% of all tweets, it's 50% of all tweet impressions. That is, multiply each user's tweet by the number of users following them when they tweeted, an

This isn't 50% of all tweets, it's 50% of all tweet impressions. That is, multiply each user's tweet by the number of users following them when they tweeted, and you'll find that 20K users (likely, people with more than a million followers who tweet with frequency) are half of all these impressions. However, the number of tweets you'd get by following the 20K would be far smaller than the firehose, probably a few hundredths of a percent at most.