I would assume that it is public roads, federal, state and local going by the descriptions on second link.
Private roads on farms, industrial plants, mines, gated communities etc paved and unpaved could well cover a significate area.
Most of the open land in Maine that in other states would be publicly owned is in Maine owned by companies that work logging. So they made a lot of substantial permanent gravel roads. Many of which are used by the public. But all on private land. Roads like this:

There's a fair amount of the state you can't get to on public roads. This is as good as it gets.