"Anime" is simply Japan's word for anything animated, so anything animated, from the Bionicle G2 animations to Ninjago's TV show to Naruto, is anime to Japan. Outside of Japan, "anime" is typically used to define animation specifically from japan. So, by that definition, RWBY isn't an anime, because it isn't made in japan. However, by the actual meaning of the word, RWBY is anime... but so is anything animated.
Basically, RWBY isn't an anime by the colloquial meaning of the word, but it is by the official meaning of the word. But the same thing applies to every animated show that isn't from Japan, regardless of art style.