The atlas entity covers the lands ringing the Mediterranean Sea. The Wikipedia article on the Mediterranean basin consulted for this entity states plainly that the basin is not the same area as the sea's drainage basin and includes countries, such as Iraq, Jordan and Portugal, that a naive coastal count would miss, so no single population total was found that matches this atlas's own definition.
What would resolve this A reference that states population for the exact set of lands this atlas describes as ringing the Mediterranean (southern Europe, the Levant coast, the Maghreb and Egypt), not the sea's hydrological drainage basin.
OpenMediterranean Basin (Wikipedia)