I believe he's creating the kind of music he would enjoy listening to, which is not necessarily the kind of music he ought be creating - perhaps that is a better wording.
Many musicians listen to music which is absolutely different to the music they are creating at that time. For example, In the Nightside Eclipse is just Thou Shalt Suffer, done in a style which better fits the music itself. It's very different to Mercyful Fate, often quoted as Emperor's main influence at the time of recording.
The interpretation of music may change as it is created - that is, the style in/by which it is portrayed by the musician - but the music itself comes from the same people with the same drive and reasons to create music as they had originally. There are definable shifts in style in Darkthrone's music, but the most notable of these, in my opinion, is Total Death, when, instead of creating good Black Metal, they created good Darkthrone.
Darkthrone are going through a midlife crisis. Well, Fenriz is, regardless.