I do see your point about the errata on Mimic, although the spell description for Grow and mature specifically state "this model", not referencing "this Terror Tot" or "this Young Nephilim".
Last I checked, "this model" specifically references the model performing the given action.
In all fairness, given the versatility of a Doppler, it now seems a bit foolish to Grow one to a Young neph, and even Maturing one to a Mature Neph might be questionable.