You are right. bitly did tracking and analytics. But Twitter can have even more information by forcing all users to use t.co.
The point of the article is that Twitter can force every one to use t.co on its platform and thus provide better analytics, malware detection etc than its competitors.
It is a game changer for Twitter monetization; not so much for users or content creators.
It seems they're planning a soft-force: all URLs will be converted to t.co in outbound tweets, but the original shortener will still be visible in some presentations, and t.co will still redirect through the original shortener -- so sites providing some unique value to users still work.
I can't see a major user revolt over that approach -- even though in the long run it gives t.co such an advantage that other services may whither away.
Alistair expects us to believe that bit.ly was sharing no tracking / analytics information with twitter?