I don't think he's using attachment in the clinical sense. He's talking more about what we think of as attachment in the lay sense, and the kind of attachment he thinks is problematic in psychotherapy. Also, the rest of the sentence you didn't quote I think makes it clear that he's not even talking in absolutes:

"The truth is, it’s never a great idea to get attached to psychiatrists or therapists of any kind unless you think it’s really necessary; we cost a lot, the meetings don’t last long, and we’re not all that friendly, really, judging from the way we talk about one another. Most importantly, whatever you imagine you’ll eventually get from the relationship, like unconditional love or feeling like a favored child, doesn’t usually happen."

Anne