Bianchigirll The dropouts are sliding so that even though they are vertical, the IS mount disc, axle, "everything" can be adjusted for chain tension.
IMG_1497 by
Richard Mozzarella, on Flickr
Also, the sliding axle plates can be replaced. There are through-axle options in both flat and post mount.
https://www.merrysales.com/shop?&search=sliding
But, yeah. A single ambidexterous frame with a brake bridge drilled that could accept a long reach calper like a Tektro 599 doesn't seem like it would be impossible to do. I wonder what their thinking was.