edirectorios Fidus Writer is using ProseMirror for the internals of text editing. The two are not really alternatives to oneanother -- they are two very different things. ProseMirror is only dealing with user input and document manipulation. Fidus Writer itself provides everything else around the text editing experience. If you write a plugin for Fidus Writer, that plugin will have access to all the features of ProseMirror.
The confusion probably comes from the term "texteditor" meaning very different things.