I don't think she is supposed to be Brunnhilde at all, honestly... the other valkyrie that pushes her out of the way of Hela's attack in her traumatic flashback looks exactly like classic Defenders Brunnhilde, in fact.
Now, I wouldn't mind if they changed comics Brunnhilde to resemble her, mind you... as others have stated, there are plenty of ways in which it could be explained, especially considering Brunnhilde's past history of 'borrowing' mortal bodies. However, it might be even better to introduce a new MU character along the lines of Thompson's Scrapper #142, a rogue valkyrie (or possibly even Disir) roaming the spaceways.