The fuck does Mard Geer have to do with anything? It's noted he doesn't go into his Etherias form very often anyway, 'cause he has no reason to, no one to threaten him. So he'd look human, yet he's recognized as a demon of the Book of Zeref. Particularly since he fucking announces it whenever he meets someone.
Also, given the nature of what Zeref was attempting to accomplish, there's no indication that E.N.D. HAS an Etherias form.
And every person who's mentioned E.N.D. to the cast has always mentioned that he's a demon of the Book of Zeref. Without fucking fail. Yet no one recognized him as E.N.D. except for Zeref and Igneel--not even the guy claiming that E.N.D. was the master of Tartaros. And, of course, neither does Acnologia, who at one point had Natsu crawling on his fucking body. He does not personally know E.N.D., and it's not a bad translation or a plothole--just your own misconception.