Yes, it works. They actually had DMW there when the web version first launched, but it was quickly removed and underwent further work and changes. It has been there for a little while now (I and someone else posted about it). It does work, but response time can be slow at times.
I have used it to re-establish dining after a cancel/re-book when I could get into the new booking on the web version, but not yet there in MC App.