I have yet to see any strong evidence at all that this is Hammer's work. If someone can give me a link ill change my opinion, but with what i've seen, its much more likely Okiver, as there's two (or perhaps one claiming to be two) people both with evidence that it is Okiver.
Also, i'm not sure if Hammer did attack something else, but i've heard a lot of people say that hes admitted to it, but even so I doubt he would do it again after finding that everyone worked it out so easily, and he got banned from it as a result.
Alternatively, although I may be wrong here and I personally have no evidence to support this, Okiver has attacked the master server and RTB multiple times, he doesn't use the forums or the game (as far as I know), so he wouldn't have much of a reason not to do it. And with Hammer's recent admittance it would be a great time to attack. However, he would be a pretty good scapegoat (I think thats the right word i'm thinking of, someone to dump the blame onto), so I could be wrong.
Im kinda friendly with Hammer, and from what i've seen and heard from him, I honestly cant see him doing this. The first attack, maybe. Lots of people do something dumb for 'lulz' or out of rage, then later regret it.
Against all this though, the HamHost servers have been attacked a few times before. With what Hammer might of said on that forum somewhere about testing a program against his own servers, it could be that hes doing his own attacks, then trying to blame Ephialtes or Lub/Rub/Whatever hes called now-a-days.