This is not true at all, except maybe in very specific places (e.g. some jurisdictions do not allow you to call yourself an engineer without a specific qualification).
Software Engineer and Developer are essentially interchangeable terms and largely a matter of taste.
Sets are super useful for all sorts of stuff where you want the datatype to guarantee there is only one instance of a value in the collection. UI components where you are incrementally adding things to a selection is a great example.
It's a completely standard feature in many stdlibs and has nothing to with turning JS into a database.