Additional to my last post, to clarify the context:

...So the sentence, "Yalom comes from the existential/humanist tradition." is not contrasting humanist with an opposite of humanist. Rather, it is saying that Yalom comes from the humanistic approach as opposed to the psychoanalytic and the cognitive/behavioural.

It's easy to miss just how massive those last two approaches in psychology and psychotherapy are and have been throughout the history of the subjects. They were the first. The humanistic approach is relatively recent and is called a "3rd force" in psychology.