Magnesium is very important but of course the big confounding factor is that a higher magnesium depletion score is also a good indicator that you're consuming more processed foods and fewer whole foods.
It’s not clear to me which foods you would need to “only eat” to not have enough magnesium. Meat, vegetables, and grains all contain magnesium. I wonder if you process meat into (for example) hot dogs, you are eating meat mixed with grains, so the grain content might decrease the total magnesium.
Perhaps a high calorie diet (soft drinks, potato chips, etc…) would lead someone to eat less “real food” (meat, vegetables, and grains), and thus overall magnesium would be less.
But still, I don’t know what one would have to focus on eating to avoid magnesium.
I don't know but I could imagine the extra oxidative stress and inflammation etc. might mean your body would need more of certain minerals than it otherwise would? Just speculating, not a doctor/nutritionist/biologist.
Edit: Processed food definitely lacks magnesium to be clear, but it's interesting to think about how it might even be more costly than it appears.