Twitter is a platform built for inclusive broadcast (to everyone), and to teenagers it offers no obvious value.
The author does not seem to realise that the Protected tweets feature is an option that obviates those objections, and makes Twitter function in the "circle of friends" manner the author desires.
What do protected tweets do that a Facebook status update wouldn't? Public tweets have added value over Facebook statuses for some people, but I don't see where protected tweets would.
Defaults matter. Twitter is public by default and the whole ecosystem is based on this assumption, the private setting is just an afterthought. Sure, you can tweet privately, but then you lose most of the features, and I wouldn't rely on security of that.
The author does not seem to realise that the Protected tweets feature is an option that obviates those objections, and makes Twitter function in the "circle of friends" manner the author desires.