"Dobrý večer" ("good evening") is used as a greeting during the evening and night parts of the day. "Dobrou noc" ("good night") is used as a wish before going to sleep, so no, these are not interchangeable.
These phrases are directly adopted from the original DL courses. The point is that there is too few of them so they are combined to have more exercises. This was not invented by the creators of this course.