I think the problem is that we're mixing together viewfinder magnification and field of view. Viewfinder magnification has to do with how big something appears in the finder compared to how big it appears with the naked eye. Most viewfinders specify their magnification assuming a 50mm lens is attached because that was long considered the standard (most common, whatever) lens. For objects in the viewfinder to appear the same size as they do with the naked eye, you would need %100 magnification in the viewfinder. But even full frame cameras are only speced around 70-75% magnification with a 50mm lens so I'd be hesitant to say there's anything special about that particular focal length. The only thing really special about it is that Oscar Barnack happened to have one laying around his work shop.
With APS-C cameras, the viewfinder is basically cropped to match the size of the sensor, so you get the same magnification in the viewfinder but a smaller field of view. If you want a normal field of view (like a 35mm lens) then you get much less magnification in the viewfinder. This was one of the big complaints about these cameras when they were first introduced because everyone was used to the big bright finders in 35mm film cameras.
So magnification is what lets you open your other eye and see things the same size as you do in the viewfinder, and that's related to the lens' focal length and the camera's viewfinder. Field of view is related to the lens' focal length and the format of the sensor/film.