I think you misunderstood their response. They are saying that the study has an unusual definition of "active", and that your need to clarify the definition proves that it is unusual.
Though personally I think filtering specifically for users that actively send tweets makes sense, since that's really what matters when it comes to measuring how healthy and authentic the discourse is
It seems like everyone is arguing about different metrics and it makes more sense to discuss different, specific measures that might fall into a range of behaviors that are "active" in some sense rather than focusing on which definition of "active" is somehow the best one.
What would be more interesting would be to adapt this and answer several different questions about the proportion of spam among accounts with different metrics of activity to see how things change. For example, does the percentage of spam accounts go down a lot if we lower the bar for "active"? How much & how fast?
Twitter's quarterly earnings define active users thusly:
> Twitter defines monetizable daily active usage or users (mDAU) as people, organizations, or other accounts who logged in or were otherwise authenticated and accessed Twitter on any given day through twitter.com, Twitter applications that are able to show ads, or paid Twitter products, including subscriptions.
I'm pretty sure I've heard a similar definition from Facebook.
This definition supports g-clef's critique that the article picks an unorthodox way to measure active users, resulting in an inflated percentage of accounts being measured as spam/fake accounts, vs what the percentage would be if measured against Twitter's definition of 'active', which includes lurkers.
Though personally I think filtering specifically for users that actively send tweets makes sense, since that's really what matters when it comes to measuring how healthy and authentic the discourse is