I think the point is in comparison to some animals, we don't extract the full amount of nutrition available in plants (which requires a much larger gut and more energy digesting).
We generally don't eat the toughest parts of plants either. Seeds and fruits are not as hard to digest as many stems or plants like grass. Not to mention that we pre-process a lot of the vegetable matter we eat - either by grinding (e.g. flour) or cooking or both.
Yes plant based food is often ultra processed and highly refined, which is typically what foodies argue is bad (except when it comes to their plant food).
