First of all, you are quite right for pointing out that the style-de-jour is 'minimalism' for app and web designers. I too feel that there is something missing from the current attempts. It is my understanding that Minimalism is a process, a process of reduction, distilling a design to its core. It is this process that produces the simplicity that often goes hand in hand with minimalist design work. However, I don't see much evidence for this process in the current fashion. I see short-cuts taken to a determined end.

The UberWriter app the OP referenced, to me, smacks of this. It appears that the end design was determined first rather than a process of reduction being made. It appears that features were decided to be unnecessary. I mean, the app requires you, the user, to determine the visual structure of the document via mark-down: the complexity has been moved from the app to the user. All the features of a regular, equivalent word processor are still present in this app, it's just you have to memorize a bunch of special syntax instead of the location of a bunch of special icons. The app doesn't bring the fruits of minimalist design and is far, far from simple.

So in that sense, I am in agreement; the fashionable Minimalist style prevalent at the moment is only skin deep. However, there are some software designers, including myself, who see it as much, much more.