It’s easy to disprove.

Go look at what tribal people eat. There’s plenty of vegetables in their diet. The only tribal people that subsist on near pure protein diets are eskimos, and even they eat seaweed.

Now, where they have a point is the mass cultivation of vegetables that don’t exist in nature. Could be something there - maybe those are not as nutritious as wild varieties, or perhaps are even harmful (eg cause inflammation) in some way.

Or perhaps genetic differences in how people absorb food and metabolize nutrients play a role in the effects of certain foods on your body. There is no absolute best, just a best for you.

I would agree that people eat too many carbs though, you can do away with almost all carbs in your diet and be just fine.

It’s certainly complicated, there’s no universal answer.