You have citations for this? I find that really surprising, and I'd like to read up on it. Almost everything I read that isn't on my computer monitor is in portrait mode. Narrow columns with lots of linebreaks. Books, magazines, comics, my e-ink kindle, you name it. The one exception is highway billboards.
Also, what exactly is meant by longer and shorter lines? It seems to me like there would be a range of line lengths that make for efficient reading, and going outside of that range in either direction, too short or too long, would result in decreased efficiency.