I think one of the volunteers in this course is someone who is involved in English academically, THINKS he speaks very good English, (but really doesn't) and feels just a little superior to everyone else because of it. Just judging from this lesson, which NONE of the other courses I've taken has felt a need to include, and the WAY overly explained tips and notes throughout the course. It's like reading a self published book that had NO editing because the author felt that no one else could possibly understand what they were getting at - there is real content here but it's almost impossible to get at it.
If none of the other courses use idioms, the only excuse I can think of for that oversight is that Duo seems to offer beginner course levels. As far as I know all languages make use of idioms -- a series of words that has meaning different from the literal translation -- so it is helpful to be aware of these phrases in a language so the learner isn't in a different "brain space" from what the native speaker might have meant.