The editors of the dictionary you're citing feel that the phrases they describe as idioms have become permanent parts of the language and are used by highly educated people in all kinds of situations. They believe that such people would not use the other phrases or would use them only in casual circumstances.
There is no firm dividing line. The current editors' predecessors would have described many of today's idioms as colloquial, fifty or a hundred years ago.
Student or Learner