Not quite the same thing. The humanoids that populate the Culture belong to any number of entirely unrelated species that are capable of interbreeding through

Not quite the same thing. The humanoids that populate the Culture belong to any number of entirely unrelated species that are capable of interbreeding through deliberate interventions (upgrades, really) that paper over the genetic, proteomic, biochemical, etc. differences. None of those species are related to Earth's Homo Sapiens, or to any other species belonging to Earth's biosphere (though I think there might have been an allusion or two to panspermia insofar as origin-of-life goes). So. When I say that Earth isn't part of the Culture, I mean to include anyone who has any Earth ancestors. OTOH, the Culture has had observers, agents, etc. visit Earth (after being appropriately retrofitted to resemble Earth humans at least anatomically, and optionally biochemically and genetically) on occasion, to determine whether to continue with the policy of nonintervention. In the Culture books published so far, the policy holds.