Twitter does absolutely nothing to exterminate old fake accounts. Probably because the last time they did so and saw how many real accounts there were, Twitter's stocks plummetted down
I think it was in a video with to mscott where woman from twitter talked about this, yes false positives are a big problem when detecting fakes.
But Twitter is working hard to keep bad bots / fake accounts away. There was no last time, right now people are working on banning fakes. It a 24/7 game.