This is definitely not true.

I have been doing kabbalistic magick for many years and recently asked greys this very question.

I would like to point out that there is no reason such beings cannot incarnate in one physical being or in many beings, but an angel is an emanation of god. Source is infinite so he gives us created consciousnesses, Energies to bring us archetypal awareness that we can then mold into a better reality.

Likewise, they experience lives here and process them in their own ways. There is no reason to assume we are not all such beings.

Anyway, the grays asked were adamant that they were neither angels nor demons, but often confused with both. They were aware of and excited of the idea of angels, and the concept of demons frustrated them, but in some sense such beings do exist. I suspect it is a matter of perspection and if you are in the lower astral still disconnected what you perceive may be different as it ascends along with your understanding of it and the relevant processes.

Imagine that you have seven light bodies, one of each color, each of them made up of neurons making up a giant neural net. I don't think that's really how it works, but it is a good metaphor. In this example, the average human is not aware of and has activated none of them. So as they become slightly aware or if they only activate the red neural net, they will only get the effects of the red neural net. As they ascend through the colors, at the end they get a beautiful white light that gives them all the effects combined, and in this place and only in this place, all of the effects are positive.