Like most of them its a reader rather than an editor. It goes by the name Das Referenz. The designers explain their ideas here. I haven’t tired it out directly because I don’t own a tablet (2015 and still using a laptop? I know).
Mostly its pretty harmless. Some messing with colours and fonts and desaturating the images (I do wonder how well that works with some of our diagrams). One neat thing that they did do with fonts was chose a specific font than Courier for the programming source code examples. They went with CamingoCode which looks solid enough from what I can tell. They don’t mention doing anything with with our equations or musical notation (in fairness musical notation via LilyPond is a fairly recent edition)
Shortening the length of the text lines is pretty standard. They shift all the images into the side column and I wonder how well that works with some of our image heavy articles.
They found our tables hard going but so does everyone (for example it is one of the long standing issues with the visual editor and flow).
Their modification of the search system was rather neat. Making the longer articles taller in the search results may make information more findable although it would need some testing given wikipedian’s habit of writing really long and detailed articles on narrow topics while the more meta articles are often neglected by comparison (generally because they are really rather hard to write).
Overall its not something I would use (I don’t own a tablet and I want to edit) but it is interesting to see their thought processes.