It's the fact that they say "Octarian" is a nationality. It's actually not, it's a species, much like how Inkling is a species, and how Salmonid is a species. Shiver and other Splatlandian Octolings are still Octarians, just that they hadn't experienced the same events as the Inkadian Octarians at all. So yes, Splatlandian Octarians do exist. In fact, the tentacle variants might just be in hiding considering the New Squidbeak Splatoon moved their operations into Alterna, which is near Splatsville, that's because they are aware of the Great Turf War.