There's literally a definition of under-represented groups in that article. What are they hiding? The whole point is that raw intelligence on some absolute scale (however measured) is not the only factor in admissions.
Their definition states which groups they consider under-represented, but not the criterion for under-representation. That this criterion is total US population (and not e.g. population with SAT score worthy of Stanford) is implied, not explicit.
The implication may be clear to you and I, but the post I replied to was literally asking what over-represented means (what, not who). Presumably they would ask the same of that Stanford article.
