Of course, being drunk isn't the only way to be imparied while driving. Being stoned, which is the point of the article linked, is an obvious other way which isn't easily testable in the field. Other ways can include medications from Nyquil to muscle relaxants, being sleepy, and so on.

In my opinion, if that cop thought you were not fit to drive but gave up after a failed breathalyser test, he's not doing his job. Of course, it's more likely that he was just harrassing you because you did some minor stupid thing on the road and had no good reason for pulling you over at all, other than that it was quote time.