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If we take Mongolia as an example, the traditional diet during the summer months consists mostly of milk products. Not sure if that counts as carnivore. Meat is mostly consumed during winter. The more northern parts of the country have a variety of berries and also some vegetables such as Altai onions (Allium altaicum).
But more importantly, most cereals and vegetables that can be grown in Central/Northern Europe can also be grown in Mongolia. The vegetation period is similar and the main limiting factor is availability of water. Nowadays Mongolians produce considerable amounts of cereals and vegetables on their own.
Of course that does not yet mean Mongolians grew cereals in the past. However, at least one study suggests that from the times of the Xiongnu to the times of the Mongol Empire, millet was an important part of the diet in what is now Mongolia.