How so? The race itself is represented as a feat. Other feats have the basic dusk elf feat as a prerequisite. Since PC's and NPC's use different rules, there can ONLY be PC dusk elves because NPC's do not get feats and cannot choose to spend one on
being what they were born.
Since PCs and NPCs use different rules, there can
totally be NPC dusk elves because NPCs
don't need feats to mechanically represent their origin. Just like if I really want an NPC to use a Spiked Chain, I can just declare it so, rather than needing them to take a feat for proficiency! Seriously, you can't use "NPCs and PCs use different rules" as an excuse to
limit how NPCs can be designed.
What happens if the PC takes the feat at a higher level?
1) The player takes the feat, gaining some mechanical benefits to represent a character that, from the start, they had played as a Dusk Elf. And no one even notices the difference.
2) The player realizes that the Dusk Elf concept really fits his character, and takes the feat, and says, "Hey guys, my character has actually always been a Dusk Elf, and he just never really talked about it until now." And everyone acknowledges it and moves on.
3) The player decides that a Dusk Elf would make a really cool character, and takes the feat for their character (currently a Wood Elf). He asks is everyone will pretend nothing weird happened in game, and assume the character has always been like that, for the sake of letting him continue to enjoy his character. The DM works out any minor inconsistencies this might cause in the plot, and everyone moves on.
4) The player things the feat will give him a powerful mechanicaly benefit, and so takes it purely for optimization purposes. The rest of the table never even realizes the character has the feat. If it does get noticed and people think it a weird feat for him based on the character's behavior or background, he comes up with some random explanation for it. And everyone moves on.