When the Beyonder gave Rachel the full power of the Phoenix the only example of Phoenix was it's mortal manifestation of Jean, nobody knew about White Crown Phoenix until Jean reach that level of her evolution. The Phoenix does not even manifest on the corporeal plane without an avatar or host, so to say the Beyonder had any notion of it's full power is a little shaky, he knew how powerful the first Phoenix was and that level of power was stated to be Jean's full potential as a psi actualized it was not the full Power of the Phoenix.