The Alternian Empire supposedly conquers aliens, but do we have any confirmation these aliens even exist in their universe? The troll kids have been indoctrinated enough to not be surprised by meeting aliens, but we never see or even hear a description of any, except the ones from actual other universes, which is weird because one would think they'd use alien slave labour or eat them or something. It wouldn't be OOC for the regime to be lying about the aliens, would it?
We’ve only ever seen Alternia. Alternia’s a space-faring intergalactic empire, if they spread outwards from Alternia and have conquered as much of the Galaxy as is implied, Alternia sits squarely at the centre of said Empire. Have you seen how brutal they are? They already have their slave labor on the lowblooded members of their own society, they likely just. Destroy everything in their path with no prisoners taken. Aliens would be much more likely to appear on the outer rim of their System than going right to Alternia. There’s really no reason why the regime would be lying about Aliens, when we do know the only reason Condy couldn’t make it in time to stop the Vast Glub was because she was conquering planets really far away, and she’s got enough expertise conquering alien species to single-handedly conquer Earth with just herself and fear.