I think it's left open for a reason. Every viewer is left to make his or her subjective conclusions based on what they see. For you, elements about them seemed divine, suggesting that they were angels. Many people assume they're aliens, focusing on the teleportation we see at the end, and the glowing object moving through the sky shortly after, heavily implied to be a ship. Still others like myself postulate that they may even be advanced humans, given their knowledge of how Earth specifically works. I don't see there actually being a wrong answer here. Rainbowfaces are mysterious creatures, and we may never know their canonical origins. But it's always fun form our own opinions.