Sure, lack of nutrients will stunt the plant in some way, and will reduce the production of some plant compounds. But the original comment's logic is just too simplistic. You can't then safely jump to the conclusion that the lack of nutritional value in plants is primarily due to the reduction in these plant compounds

The main drivers could very easily be due to a variety of other factors. Like the sibling comment says, it could for instance be changes in agricultural practices (like the plants growing faster) b/c farmers naturally optimize for volume and not nutritional value per-kg or per-calorie

There are just a million other variables at play - and the nutrients in the soil aren't intrinsically the limiting factor for the nutritional of plants - b/c what's nutritional is fundamentally different