My account is one of those 44%. I have nothing to say on Twitter, but having an account lets me follow the people that I want to. This still makes me an active user, though.
Logged out without sending a single tweet and never went back. I still get a few emails from them every month wondering what happened to me. Still can't figure out why I would need an account.
Unfortunately, there is no way to measure this case, except for counting the people you follow or the people who are following you. Then you could know whether that user is somewhat active.
This data wouldn't show on outside measurements (like in the article) but would help Twitter know who is active and not.