I'm not saying that a bulk of their calories came from non-animal sources... the bulk of most meat eaters' calories comes from non-animal sources. That said, I find it entirely improbably that they were abstaining from meat.
Abstaining from meat isn't a new thing though. In many historical cultures banning meat from certain groups was enforced as an indication of status for those that were allowed.
This is suspected to be part of the reason for the neotenic expression of blond hair and blue eyes that emerged in Scandinavians, as you would normally get enough vitamin D from meat sources, so the evolutionary pressure for very fair skin is presumed to have come from the lower strata of society trying to live off grains and other vegetables while getting very little sunlight.
