Every build has its foil. If this guy is suggesting Assassin for its kill-sealing ability, he's probably basing that either on assassinate or stalked up trance. In either case, your target has to to not be able to see you, which makes duergar pretty resistant.
That an exception exists doesn't <i>necessarily</i> mean a given build isn't good at sealing kills. It may just mean that build is only good at sealing kills <i>in certain circumstances</i>.
Take orcs. Orcs are great for sealing kills against classes that lack bash protection and that an orc matches up well against in melee. They're not as great for kill sealing against people with bash resistance. Does that mean orcs are categorically "bad" at sealing kills? Not really. Just means they're "sometimes really good" and "sometimes not so good".