The UC’s have 2 additional comments section. Both require a written explanation and there is no indication you should be linking documents or websites. I would follow the UCI reps advice.
Academic Comment section:
Admission staff look at this section for an explanation of abnormalities in a student’s academic record, such as a break in attendance or poor grades in a particular course or year, or for specific information about the school environment/policies that affect the academic record and/or choices for classes.
Additional Comment section:
Additional comments (550-word limit) in the Personal insight section, which can be used to address anything else that is important for you to mention. If anything you have to explain about your academic record is too long to fit into Additional information, put an abbreviated explanation there and refer the application readers to a longer explanation in the Additional comments.