The argument doesn’t need to establish this, because the conclusion is only about high school graduates.

This possibility points out why attending university can still help high school graduates get a corporate job, even if they already have the attributes most valued by corporations. Perhaps the corporation requires a college degree, for example.

The author assumes that people who already have those attributes won’t increase their chances of getting hired by a corporation by attending university. This doesn’t require that corporations only hire people with those attributes.

The argument doesn’t make any assumptions about the high school graduates’ purposes in attending university. The argument is only about the effect of university on one’s chances of getting a corporate job.

The author doesn’t assume that these attributes can be acquired through study. The author relies on high school graduates who already have those attributes. Whether they got those attributes through study or not has no impact.