I do think it is impossible to completely understand certain musical themes unless you have some base of knowledge to build off of, but this base is NOT only created by playing music.
The musician having a deeper grasp on music argument may hold a little water, but it is not true to the extent that being a musician is necessary for true enjoyment and understanding of music.
Furthermore, to critique music, you do not have to be a musician and to even state this is ludicrous. The nonmusician opinion will actually be more agreeable to more people as musicians are not the majority. To critique a certain level of skill could be helped by the critic being a musician, but not much. For an analogy, does an amateur movie critic need to create a movie in order to criticize this popular form of entertainment? Must one create a video game before realizing that a certain video game is crap? Of course not, and the same level is easily transferred to music.