The NSA has nearly unlimited hiring potential with just the pool of military folks alone that work there. There are constant hiring freezes for a year at a time, because a lot of buildings are over capacity.
If one of these college students were to apply to the NSA through NSA.gov (mandated as the only way someone is allowed into the agency), they would have to apply a minimum of 2-4 times, because the application stays good for about 90 days, and the average applicant waits 6 months to a year to get accepted. This is assuming they're applying straight through without having someone with hiring power pull their application from the queue. After that, the process to get a clearance could take anywhere from another 6 months to 2 years (possibly more in some exceptional cases) and costs the agency about 250k.
So, it goes without saying that bringing onboard a fresh college student who isn't going to even get to walk through the door at least a year after applying, can pale in comparison to bringing on someone who already has a clearance and training. The article assumes that the NSA relies heavily on its external recruitment, but the vast majority of folks working there just change out of a uniform into jeans and a shirt.