And they didn't eat those things except as flavourings for their meat, medicines, or emergency food when the hunt failed.

Such a marginal consumption would not leave relevant record in fossil findings.

No one ate much plant matter except for the week when local fruit fruited until they settled into villages around farms when agriculture reached western Europe 4000 years ago.

There are enough findings pointing to the collection and consumption, by humans, going back to 100.000 year.

My ancestors are from that region, grain intolerance is common in that population as we haven't had it long. Cabbage is fine as we had that before we started farming it

Where? I'm in Portugal and there is no specific intolerance to grains in our population.

Brassica plants are harder to digest than cereals, even when cooked. Much of the fiber passes through the digestive tract untouched. The nutrients extracted from it are mostly water soluble and only after cooking.

Things are like that all over the world, people got agriculture 10kya (kiloyears ago) in Egypt through 200 years ago for the first contacted indigenous Australians, to not yet for some uncontacted tribes.

The indigenous populations of Australia already practiced a form of agriculture prior to colonial establishment in the region. Many areas used for that sort of farming was destroyed by the introduction of animals brought by europeans and that caused serious conflicts between the settlers and indigenous people.

The less time since introduction of high dosage of plants and the poisons they carry on introduction of agriculture the less will those populations handle those plants. And none of us handle them as well as a herbivore. At best you use extra vitamins and iron to detoxify the plants you eat, at worst you're intolerant.

I can't find it now but there is a protocol developed by early peoples and still used today, to a degree, to test new potential food sources. Early humans were not more stupid than we are today and no food source was added without careful experimentation.

I can't understand the use of "detoxify" in this context. Most potentially harmful substances in plants used for human consumption are usually removed through preparation and cooking. Genetic intolerances are not derived from toxic substances in the plants.

The only food reliably available in snowy conditions is meat

Majorly, yes. Other sources can be found but are not as calorie dense and the effort required to harvest it can make it calorie negative.