They ought to be. RAW (rules as written), neither the description of a duergar as a monster in 5e nor of a duergar as a player character in SCAG says anything about their psionic powers, other than the magic that comes naturally to them. Some choices could be made up by the DM. The duergar, on the other hand, took a different road than the githyanki. Even though the githyanki hated the mind-flayers who used to be their psionic masters, they decided to use the power that came with having worked with them in the past. The numbers they have show that. The duergar, on the other hand, decided to escape, be negative, and feel sad. Instead of enjoying psionics, they fight it. This is mirrored in their resistance to magic, and if I were the DM, I would also make up some resistance to psionics.