Well, this is really the type of thing that should go under the "questions" category, but I'll go ahead and answer here....
At the time Cas said archangels were absolute, he didn't know the leviathans existed. They are older than all angels, and really all currently known creatures except for God and Death. Angels couldn't kill them any more than the Winchesters could (as Death put it, God feared they would "chomp the whole petri dish"). Leviathans trump angels.
Plus, the garrison was saying there are very few angels remaining after Castiel went all Glen Close on the Raphael supporters, so there may not be any archangels left...but that last bit is just my insinuation. Hope this helped!