Being a seedling tree does not increase the cold hardiness of a variety, but as you state it does frequently protect the loss of a cultivar by regenerating the variety through a renewed growth from the root system. Trifoliate root stock being incapable of supplying enough water during a severe freeze, could indeed be true. However, during a severe freeze (<55.4F root zone temperature), every variety of citrus would have a hard time supplying water, or doing most anything physiological for that matter. - Millet (668-)