>They talk about "active" accounts (meaning have tweeted in the last 9 weeks), and do a bunch of filtering against that. That seems like a huge bias - lurkers exist, and in my experience are usually the majority of users...this step removes them or ignores them entirely.

All true. However, do you really believe that a bot is more likely to be active than a real user? If so, fair play to you. If not, then we would expect inactive users to be bots in an even greater proportion than what we see among active users.