Given that each planet seems to have one species, except that humans live everywhere, I always kind of assumed that the Galactic Empire was one with humans at the core.

That is a possible reading, especially given the way the Empire's xenophobia manifests, if you assume all humans are culturally homogeneous in Star Wars, but I think the movies are somewhat clear that humans are culturally diverse. The human cultures in Tatooine seem very different to Alderaan or Naboo, for two somewhat specific notes in the films. The Imperial accent often being more "British" (RP) implies cultural differences with the periphery that at least mirror somewhat, say, the differences between Britain and much of its former Empire.

The Galactic Empire showing a clear citizenship "caste" favoritism of Citizen/Officer Humans, General Humans, Cloned Humans, Aliens, and then Droids, but also working with all of those at the same time in its military efforts (there are a few aliens in the Empire's military, there are a lot of droids) does seem imperial in the way the article suggests, especially because that core of clear "Citizen/Officer Humans" seems so relatively small versus they armies they command of all the other types.