inadequate dietary Mg is prevalent in America and is the most likely cause.
It contains zero qualifiers suggesting it is one of many factors, a major factor among many, etc. It uses the word "the" and the singular form of "cause."
You skipped right over "most likely" and only focused on "the ... cause". To me it sounds like they are saying statistically Mg deficiency is the most widespread underlying factor. Their language is strong, but it does not come off that absolutist and the "most likely" suggests other causes.
I'm a freelance writer by trade. I sometimes write health articles. I could not say what this article says about cause.
At best, I could say something like "Given the proven widespread lack of adequate dietary magnesium and the known side effects of magnesium insufficiency, it seems plausible to suggest that at least some portion of the increase in depression is due to widespread magnesium deficiency."
That may seem like quibbling to you, but scientists get paid to quibble about the details and to be as precise as humanly possible. They should be meeting at least as high a standard as my generally low paid freelance writing.
And of course, not everyone who gets too little mag/too much calcium will react in the same way.