Who says it is fabricated?
If you look back through history, from The Bamboo Cutter to the tales of gods and goddesses - the Epic of Gilgamesh, etc - all of it is pretty clear that other beings similar to us exist.
Consider that in the lore of the 40K universe, there are planets part of the imperium who do not even know (or remember) they are part of humanity.
Though, honestly, I have had my finger over the order to carry out Exterminatus. Too much heresy around here.