I think it's instructive to note the difference between 'fake' accounts and 'spam'/'bot' accounts.

'Fake' accounts exist to fraudulently increase follower count. This is what the study is claiming to measure. These typically have low activity and engagement profiles.

'Spam' or 'bot' accounts, on the other hand, generally have high activity. Whether trying to influence political opinions, or engaging in astroturfing or phishing activities. They probably have very, very high ratios of replies to original tweets, and overall tweets to # followers.