UIUC does not consider level of applicant’s interest (yield protection) in making admission decisions. So the idea that a lower qualified applicant has a higher chance of admission for yield protection reasons is dubious there.
Note also that UIUC admits by major, not just by division. CS appears to be the most oversubscribed major in UIUC engineering, so it is likely to be significantly more selective than other UIUC engineering majors.