Their academic ranking was on the rise before they became nationally prominent in football. Otherwise I agree with what you said. Similar to Va Tech.
They are strong in the STEM disciplines (science, technology, engineering, and math) whereas USC is strong in everything else (business, law, education, journalism, nursing/health sciences, political science, criminal justice, behavioral sciences like psychology and sociology, and fine arts like music and dance). A more specialized school that focuses on the STEM disciplines is naturally going to have higher average SAT scores and tougher admission requirements. But that doesn't make it a "better" school. It all depends on program of study. You want to be an engineer, architect, or IT professional, Clemson would be the better choice. You want to major in almost anything else, USC would be the better choice.
And by the way, that difference is intentional. Most states have one flagship public institution that offers a broad, general studies education, yet another that has a more specialized focus on STEM or agricultural disciplines. Consider UVA vs. Va Tech, UNC vs. NC State, UGA vs. Ga Tech, Bama vs. Auburn, Texas vs. Texas A&M, etc. Typically the general studies university was founded first and the sibling STEM institution was founded later to serve that specific need. It's a very common model in public higher education.