medium natural humanoid, dwarf (devil)
That's the keyword line for all duergar entries in MM2.
Contrast this with:
medium fey humanoid, eladrin
which is the keyword line for MM2 eladrin (though admittedly MOST of them inexplicably lack the eladrin part, probably should be errated).
Interestingly in MM Drow have just:
medium fey humanoid
and no racial component at all.
I think not too much can be concluded from keywords going by that. Seems their application is rather haphazard. In fact MM1 and MM2 are neither consistent with eachother in the definition of the format of the keyword line, nor do they seem to internally stick to their own formats. According to MM2 you should parse the duergar line to mean:
size -> medium
origin -> natural
type? -> humanoid (seem to be missing a comma here)
type -> dwarf
keywords -> devil
Thus duergar are of "type" dwarf. What exactly does a type mean? It isn't actually defined ANYWHERE in the rules, so I guess it means nothing. Essentially ALL of these things are simply keywords, but since they aren't mapped to things in the PHB like race, class, etc in most cases one must assume that by RAW there is no such thing as a duergar "race". Of course PCs are different and presumably following the MM2 race template makes you Race: duergar (yeah, I know, duh!). Anyway, duergar are "sort of" dwarves the rules. They might potentially be affected by things that affect dwarves but there's no basis in the rules for saying they are of the dwarven race, nor that monsterous duergar even exist as a distinct classification within the rules.
So no dwarf prereq feats by RAW, but I think this is not news. I'm not convinced that they should have dwarven feats either. I'd be more tempted to just make up some reasonable racial feats. Maybe stealing a bit from tieflings is a good idea, but also doing something with their quills is already included, so maybe a paragon and an epic feat to go with that? I'd model those on the dragonborn breath feats.