Yes, very true that the student in this situation can choose to stay 8 semesters / 12 quarters or graduate earlier. However, students who take college courses while in high school often have not been choosing them for the purpose of fulfilling college subject requirements (major, general education, etc.), so two years’ worth of such college courses may not necessarily advance a student two full years toward graduation, particularly in majors with long prerequisite sequences or courses rarely taken by high school students.
But note that at least some UC campuses do not prioritize registration purely by class standing. Some courses use class level (which counts semesters attended since frosh entry); some lower division courses prioritize frosh/soph undeclared students who may need them for possible majors, so junior/senior students taking them for out-of-major electives may have lower priority.