A horizontal split image needs a vertical line to function. But that line does not have to be straight up, it could be at an angle. But you are correct, w/o a vertical line to split the image, you can't really use the split image rangefinder. Well you could rotate the camera about 30 degrees, so that the horizontal lines are at an angle in the viewfinder.
If you rotate the screen 90 degrees, so the split is vertical, then you can use the rangefinder on horizontal lines.
However, this is a PiA do to, since rotating the screen is not a task that I would do in the field.
This is why I like the 45 degree split image rangefinder on my Nikon. I can use it on both horizontal or vertical lines.
The other option is the microprism. Some like it, some do NOT like it. I was brought up on it in in my first 35mm cameras, so I like it.

If you shade your head, you can see the image better to focus. I had this exact problem a week ago. It was bright and the sunlight was making it difficult to just see the screen never mind focusing. I was tempted to take out my LF focusing cloth to give me shade into the WLF.