... are indeed one and the same.
These images were taken from Artbook #5, the very last page, where Naoko does her "thank you's" to the fans for purchasing the book. Aside from this, if you go to The Oracle, the translations of one of the paintings states that Naoko had intended for a picture of Eternal Sailor Moon and Sailor Cosmos to be layered, one over the other, but the picture of Cosmos didn't show up properly.
On top of all that, the translation of the last few pages from the Final Arc, where Cosmos is talking to the Sailor Quartet is taken a bit too literally.
"Sailor Cosmos!?" says Ceres. "It can't be... It can't be... Are you the future... The future Sailor Moon's ultimate form!?"
"I am only a coward," the woman says. "I abandoned everything, and ran away from where I should have been. Eternally, I am no match for the final courage and strength of Eternal Sailor Moon."
"Final courage!? Then, is Sailor Moon-!? She can't be..."
"Sailor Cosmos! Our princess...!"
She never says that she is NOT Sailor Moon, or one of her forms, she simply states that in this current form, that she is no where near the level of strength of Eternal Sailor Moon, which doesn't necessarily imply that she is not her from the future.
She later goes on to say, regarding her appearance as Chibi-Chibi:"The me of here, too, was all alone. Always suffering. So I was always by her side, to support her. I was going to make her choose the right path this time.... But I knew. The chosen path was not a mistake. Nobody can destroy the place where stars are born. Because of this place, we can continue living. Time and time again, we can start over. I won't run away anymore. I'll go ahead to the future. Because I was blessed with great power by Eternal Sailor Moon. All the courage and strength to stand alone, the courage and strength to accept everything! I had forgotten it. The matchless strength!"
She is saying here that she cannot be the person she was before, in terms of courage, strength and bravery.
They are the same person, but Sailor Cosmos cannot be the same as she was before because of the effects of the war with Sailor Chaos. She doesn't have the same level of sacrifice in her that her former self did, and she had come back from the future to destroy The Galaxy Cauldron so that her terrible future would never happen.
So, what it comes down to, is that Sailor Cosmos is referring to the emotional strength that Eternal Sailor Moon has, the ability to sacrifice everything to save her friends and her future, where Cosmos does not possess that level of maturity from the future and chooses to come back to destroy in the past what will ruin her future.
(These translations were taken from Alex Glover http://www.kurozuki.com/takeuchi/sailormoon/old/act52.html
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