Quel'thalas was founded 7.000 years ago, and aside from the Troll Wars (where if I am not mistaken one night elf helped the high elves), the quel'dorei never interacted with their Kalimdor kin until their rediscovery in the Third War. Maybe she heard of the night elves in history or in legends and never believed they existed. If anything, a lot more high/blood elves should be surprised that night elves exist. I don't know what's written in their history books, but 7.000 years are a lot and history can get twisted or some major details forgotten.
Basically, I interpret that comment as "WTF, I am really fighting alongside this ancient and legendary race?!?!" instead of "Eugh, night elves are disgusting, do I really have to work with these animals?".