The tedious details that would go into trying to figure out precisely how NxT would be attacked don't resolve the problem that the paper is talking about, and more importantly, it's not the responsibility of us to put forward the security model.
but that's excatly what it does. The n@s attack the paper talks about is not feasible because you'd need more processing power than anyone could own. Prove me wrong. I still don't see any hard math in your claim.
edit: only speaking about nxt. don't know the others enough.