"Active user" is a common industry term with a well-defined meaning. It's misleading to use it to mean something else, particularly when there are a number of more appropriate choices, e.g. "20% of Twitter posters".
The article clearly defines those accounts as "active" because it's the only way an external observer can somehow isolate an "active" group. Only twitter can know how many users are "lurkers".
And since they are trying most probably to get some PR for their company, they use their specific definition of "active Twitter account".
