If you want to get really technical, everyone is an alien. You included. But hey, I was trying to keep things easy to explain so I'd appreciate it if you didn't get pedantic on me.
Where I come from, vampires can't just sire anyone there. They weren't siring other demons, and really you could probably classify them more like aliens on Earth since most of them just came from different dimensions and hell dimensions or stuff. I'm willing to bet a lot of those that people called demons actually weren't. Point is, yes, it is surprising if you knew how it worked back home.