Yes, he is the real Mattias Marklund from Vintersorg.
But, as you can see he don't post very much, so i'll answer it to you, cause if you'll wait for his answer it will take untill october or november(hehehehehehe...just kiddind Marklund...).
Oh, and if you see Tyr or Asgeir in this forum or in other on the Ultimate Metal, they are the real ones too! (The toher guys from Borknagar too, but just this two have anything related to the band... I guess Lars have added some Keyboards, right?)