It is impossible to have a perfectly flat base. The ground is uneven, the board is not perfect, the human body does not have the fine motor control to adjust pressure that precise. Now, a 50 micron section of graphene (single layer of carbon atoms) that can be damn flat!
The true problem is what we seem to term 'flat-base', I bet if you did a quick survey on the hill, most people would consider even having a slight edge flat-based ... but hell, most people think they are carving.
It's also not like there is an international snowboard standards commission that decides this stuff, cause most people don't really care that much. I think most people would accept that flat basing is where the bulk of your base is in contact with the snow and you are actively trying to keep it flat.