UCLA is strong in things other than the liberal arts. It has a top medical school, top programs in many sciences, and very strong professional schools (B-school, law school). In the sciences, physiology, psychology, anthro, and chem are all top 10. In engineering, some consider UCLA aerospace to be top 10 as well.
While it is clear that USC has a better engineering program, I don't think that UCLA is only particularly strong in liberal arts.
Unless of course you count physical sciences and social sciences as liberal arts.