When a plant-eating population has poor health, you just say they weren't doing it right
Setting aside a counterexample is only a dodge if the condition is invented after the bad result. 'Enough calories, a B12 source and adequate iron' has always been part of the claim. A population short of food struggles on any diet, including herding diets heavy in animal foods.
If a whole population has eaten mostly plants for generations and still shows deficiencies, that is evidence about the diet itself. Answering that they didn't follow it properly lets any bad outcome be explained away and any good one counted. A claim protected like that can never be tested. If vegan diets are as robust as claimed, real people eating them in real conditions should do well, not only careful, well-off people with supplements.
It depends on when and how the qualifier is applied. If the condition were made up only after a bad result turned up, the charge would stick: a claim that can't fail can't be tested. That is not what is happening here. The claim about vegan diets has always come with conditions stated in advance: enough food, a reliable B12 source, attention to iron. Those same conditions are applied when the outcomes are good.
A population living generationally on plants in an impoverished region often meets none of them. Food can be scarce. Supplements and fortified foods are unavailable. The diet may not even be strictly plant-based. Its health problems are real, but they track poverty, not the absence of meat. Pastoralist groups such as the Maasai, whose diets are built on milk, blood and meat, also show high rates of anaemia. Poor populations struggle to meet their needs whatever they eat.
There is a fair test of whether 'they didn't do it right' is an excuse. Does the good result hold where poverty isn't the explanation? Large cohort studies of vegans in wealthy countries are not made up of specialists following elaborate protocols. Even after adjusting for education and income, they show low rates of chronic disease. The care involved is mostly a B12 supplement, and that is not much of an escape hatch.
None of this makes the poorer population irrelevant. It shows that diet quality and food security matter, and nobody disputes that. What it can't show is that someone with access to a supermarket needs animal products to be healthy.