That’s sort of a general statement

which could be interpreted in a few ways. First, from the point of view of your diet, it’s best to eat a wide variety of veggie simply because they each provide different nutrients. If we all lived just on tomatoes, we’d get lots of Vit C but not much Vit A and other nutrients. If we all lived on garlic, well we would smell great but we wouldn’t have much Vit C. Etc etc.

Or, she could be talking about rotating veggies through each garden area (either through the beds or through the rows), in subsequent years so that the soil isn’t stripped of one narrow set of nutrients, but rather hosts different plants each following year (and hopefully is replentished with compost and other natural soil amendments in between)
Or she could be talking about different varieties of the same veggie, like Brandywine vs Roma tomatoes, Snow peas vs snap peas, pinto beans vs kidney beans, etc. Growing different varieties of the same veggie can result in a wider range of harvesting times, and sometimes different nutrients (some varieties being selectively bred for richer this-or-that).

I would guess she meant the first interpretation, where a variety of veggies in the diet makes for better nutrient intake overall. But maybe some of the second and third interpretations as well. A diversified farm is generally a healthier place in terms of nutrient cycling, just like a diversified diet is for the individual.