I don't know if that's true. We may not have been able to supply the amount of vegetation required to keep a human being upright and functional a millennia ago, but now that we live in the age of abundance, with food diversity that's unprecedented that may not be an issue at all. We can artificially create a complete diet of strictly plant matter with intelligent choices.
That's very hard to distinguish from supplements. Some nutrients are hard to find in useful quantities in plants, so you need a ton of them. The most practical approach is to extract and put them in a pill. Supplements. Nothing wrong with it.