These types of papers are usually published in journals that specialize in reviews. Reviews are papers that summarize research and try to make sense of it.
The folks interested celiac disease and Type I diabetes research have known about zonulin controlling the gut "sieve" size for a while now. They are starting to run with it.
Celiac patients have increased levels of zonulin in their gut, for example, compared with non-celiac controls.
This paper is a synthesis piece. They are taking research from seveeal areas and trying to synthesize an overall viewpoint. This is analogous to what the Physics community is trying to do with the many proposed models to bring all of Physics together. Think of the paper as proposing a model, one that can and should be tested extensively.
Here is another paper like your link:
It has a nice review of literature and a good explanation of what is currently known.
But you will notice the word "cancer" in the title. So again this is a synthesis piece. And is speculative.
We need articles like these. They have a useful function. But we do not have to buy what they are selling.