Eldar means elf, we know this because Tolkien invented the term and said it means elf.
While Drukhari and Druchii have a different history, and Games Workshop mostly retconned the idea that the WHFB world existed somewhere in 40k, I have fairly little doubt you could find a WHFB Dark Elf calling themselves Eldar somewhere, at least in a “Eldar is the elven word” kind of way, and culturally they’re pretty similar in theme. At least in the “BDSM reaver and slaver Elves that don’t believe in consent” kind of way.
Tl;Dr Dark Eldar works for WHFB just fine, because they stole half their ideas and terms from Tolkien anyways. The rest they stole from Moorcock.