As someone who was studying engineering at Exeter, I would say those entry criteria are reflective of the course (and my impression was that Maths and CS, which share the building, are similar).
Also Exeter doesn't take "underperforming students who haven't been able to achieve their potential", it takes upper middle class white students, many of whom were at good private schools and depsite all of this (and their weekly Oxbridge interview tutorials etc) still failed to get into a "better" university. There was an Etonian on my course, and I assure you he wasn't an exception (or exceptional in any way) to this.
Some of their departments are quite good - Physics, from experience, is quite well organised and reasonably rigorous, and I hear Law and History are well regarded (by students and people outside the university alike). Many however (including essentially all of CEMPS outside Physics and maybe Natural Sciences) are at best mediocre.
Incidentally the "reason" they're so overrated is due to very effective marketing campaigns by the university...