There is also the possibility that it does help, but not strictly with the grammar or the mechanics of the actual language. I can imagine that learning a foreign language with any proficiency is as much about learning how to teach your brain to break free of the constraints of knowing only your first language. For most people, thoughts are directly connected to language and sentence structure. Perhaps learning another language teaches you to think more in the concepts and be more flexible with your thought structure than thinking simply within the natural constraints of your first language.
For instance, if you can't wrap your head around why a language may be missing 'basic' elements, you would never really master that language.
It seems like people constantly overlook the differences between reading textbooks versus reading novels. When reading for academic purposes (including novels for say English majors), the pattern of reading is often non-linear. People may read through it once or more times, but the majority of the time is spent skipping around, constantly flipping back and forth trying to find the relevant passages for reference. Personally, this type of reading is a very visual experience for me, I remember where a certain equation or figure was on the page and can quickly flip to it even if I don't remember the exact contents. Something that is not possible in the same way digitally.
On the other hand, I would guess that when most people read novels recreationally, they are reading in a linear fashion. There is a much smaller need for readers to constantly be flipping to different sections of the book while reading a novel.
There is also the issue of where people read. For textbooks, it is pretty easy to set a large book down on a table or desk at home, in a dorm room, or in a library. It is much harder to do so if you read primarily on a train or bus during rush hour or at a bus stop.
>“If it was too salty, they would be flowing year round,” Dr. Stillman said. “We might be in that Goldilocks zone.”