But to take that and declaratively state there isn't a single non-human on the whole planet is a little bit silly.
With the Asari culture that has been open to relationships with members of different races since the first game, even to the point where the species has developed derogative slangs against 'pure bloods', it's much more plausible to believe that there are other races on Thessia than there are aliens on Earth.
Heck, if you actually read the codex entries, Earth is hardly the center point of the Alliance. It was Arcturus Station that held the high ranked leaders and was the center of the Alliance military.... Which begs the question as to why Shepard was sent to Earth instead of the station where the Alliance leaders and parliament resided.