You're over-complicating the issue, I think. Why not just do it themselves, then claim it was 'Anonymous'? No convincing required and no one to argue with.
The whole thing smells like just another move to convince the non-believers into thinking NK was responsible for the Sony hack.
Both the US FBI (via, for example, their mole sabu) and the UK (via GCHQ's JTRIG) have previously conducted offensive operations under the cover of Anons. (Of course that doesn't make a lot of sense as an attribution, as Anonymous isn't really a group of any conventional kind.)
This looks like a small DoS against one router, as any random could do (and perhaps has done), but of course, .kp doesn't have much connection to the public internet, so that's about all it takes. And it isn't exactly a big impact on them as a country - yet it's still knocking a country off the internet.
I believe the attribution to DPRK is very probably false. I think the Sony hack was quite probably conducted by non-state actors, but there is far too little information to be sure. The US have unfortunately impeded honest forensic investigations.
And I believe that attacks on the internet, any part of it, are destructive and not constructive. The US would be extremely displeased if the same - cutting it off the internet - happened to it, so I dearly hope they are not stupid and short-sighted enough to be responsible! Whomever is behind this deserves condemnation.
Perhaps we need a treaty. Yet when a few determined individuals can do offensively essentially what a nation can, I'm not sure how that's possible. But focusing on offense will only make everything worse - the US probably has more to lose from so-called "cyber war" than any other nation on earth. It should be leading the charge against it. It isn't. And that's a huge mistake.
The whole thing smells like just another move to convince the non-believers into thinking NK was responsible for the Sony hack.