The title is wrong. Buzz itself is not opt-in, auto-follow is, but not yet, and not retroactively.

The blog post also glosses over something critical:

This created a great deal of concern and led people to think that Buzz had automatically displayed the people they were following to the world before they created a profile.

But what about people who already had a Google profile, who did have their contacts automatically displayed to the world? Google still hasn't acknowledged this or apologized for it, and they continue to act like everyone who is complaining is getting upset over perceived rather than actual privacy concerns.