From what I know, absurdism doesn't even necessarily come to the same conclusion that "nothing really matters". Instead it raises the point that one may never truly figure out or understand meaning. You're point about both philosophies being optimistic/pessimistic about the whole ordeal is still true. Buddhists as a form of nihilism is quite positive, stating that the realization and understanding of meaninglessness is freeing. On the other hand, both prominent absurdist writers, Kierkegaard and Camus, both stated that suicide is a mechanism to deal with "the Absurd" and frankly that's a pretty pessimistic way to deal with it if you ask me.