The argument is that Google+ is unsuccessful, and Twitter is, on the other hand. Twitter is booming with growth and is a prime target for Google if their goal is really to own a social network with a high amount of usage and and public knowledge. Google+ is not.
Arrington presents that it would be akin to Google purchasing YouTube even when it had Google Video gaining moderate traction over a slower period of time. Twitter is to YouTube as Google+ is to Google Video.