We live in an age where people are cosplaying characters from games that aren't even out yet. Sometimes this is advertising, sometimes it's just excitement, but when it looks this good, I don't know if many people are going to care.
With help from reference materials provided by the dev team, I'm sure the outfit is "authentic", but whatever. Even if it wasn't, it still looks badass, and at the end of the day that's what counts.
There's a breakdown of how the costume was made on Hellblade's site.