Agreed, I'm not sure where the ideas of her being anything less than that are coming from.
Like many other things in Bloodborne, you can interpret the evidence in a multitude of ways. There's no hard answer.
Let's go off of the assumption that the Moon Presence and Mergo's Wet Nurse are "True" Great ones.* What does that mean? I'm not really sure, but the common interpretation is that they are "natural" Great Ones who were never human. Let's look at some of the things that have in common:
- "Nightmare Slain" when they die
- Each associated with a dream world
- Each closely associated with an umbilical cord third**
- The player is not given their true names. Keep in mind that the boss names in Bloodborne seem to be "word of God" (e.g. the player never hears "Vicar Amelia", but they know her name. Same goes for the Shadows of Yharnam, etc.). Even the descriptions of the nameless-bosses seem to be "word of God" (e.g. I, for one, wouldn't have labeled the creepy invisible reaper as a "Wet Nurse"), so it's REALLY unusual that they aren't given names.***
- Neither is a Chalice dungeon boss: their battles are unique.
None of these things are true for Ebrietas. So the question becomes: why is she different? Why was she "left behind"? Who was she left behind by? Why is she ALSO present as a Chalice dungeon boss? The same question goes for Rom.
Ebrietas means "drunkenness" in Latin. The only other reference to alcohol is in the Pungent Blood Cocktail, which describes blood as "intoxicating". Ebrietas also vomits blood as one of her primary attacks. Ebrietas is closely associated with the labyrinth. Could Ebrietas have once consumed a great deal of blood, much like the Pthumerians? Could that have been related to a possible evolution from a Pthumerian to a Great One? It seems possible.
* Oedon is a bit of an odd duck here, as usual. He/she is closely associated with an umbilical cord fragment, and it isn't clear whether Oedon is the creature's true name or a name given by the Church to the mysterious incorporeal force that lies within the blood. And, of course, Oedon doesn't get explicitly associated with a dream world or deliver the player a death message. Nonetheless, the "formless" nature (not just invisible, but without ANY physical embodiment. Oedon is a concept with a motivation. Isn't that creepy?) seems to be about as far as you can get from the more "grounded" Great Ones.
** Rom also fits all of these bullet points except for this one. She is one degree away from Master Willem, who has his own umbilical cord. This cord has references to Master Willem's dreams of evolution but none to Rom herself, unlike the cords of the other Great Ones. I'd be inclined to say that Willem's cord is of a different sort of origin than the other three cords.
*** Aside: Miyazaki said in the guide interview that he puts a great deal of care into each name he puts into the game. Take that as you will.