Plants contain toxins, and people need to know there is a safe upper limit
Every food has an upper limit, animal foods included, so this is true but not a point against plants in particular. For oxalates and lectins, ordinary intakes and normal cooking keep people well clear of harm. The reported cases involve extreme amounts or raw beans, not a normal varied diet.
Some plant foods contain compounds such as oxalates and lectins that can cause real harm in large amounts or when badly prepared, and there are documented cases of people being hospitalised. Plant-based advocates tend to present plants as harmless in any quantity. People deserve honest guidance about where the safe limits lie, not an assumption that more greens is always better.
Yes, there is an upper limit. There is one for almost everything people eat, and that is why the point does little work against a plant-based diet.
Take oxalates. The poisoning cases on record involve extreme intakes, such as large daily volumes of juiced spinach or rhubarb leaves, which are not eaten as food. Kidney stone risk is the more realistic concern. In large cohorts such as the Nurses' Health Study, dietary oxalate shows little link to stones once calcium intake is adequate. Calcium binds oxalate in the gut. The figure of 100 to 150 mg a day sometimes quoted as a ceiling is a precautionary guideline for people already prone to stones. It has not been shown to harm the general population. A cup of spinach alongside properly cooked beans is a well-studied pattern with good health outcomes.
Lectins are similar. Raw or undercooked red kidney beans can make people ill. Soaking and boiling, or pressure cooking, reduces lectins to undetectable levels. Confirmed cases are rare and rarely serious.
Animal foods carry the same kind of caveat, often with worse numbers. Liver can deliver toxic amounts of vitamin A. Some fish carry mercury. Undercooked poultry causes salmonella on a scale far beyond anything recorded for lectins. Nobody concludes from this that animals are trying to kill us. They conclude that food needs sensible preparation and some variety.
The advice itself is also not missing. Dietitians routinely tell people to vary their greens, cook their pulses, get enough calcium, and take care if they have a history of stones. That is ordinary nutrition guidance. It applies to omnivores as much as vegans. It is not a reason to keep animal products in the diet.