Hiring is particularly tricky for Valve, since their anarcho-syndicalist structure only thrives when employees are both self-driven and broadly talented. As such, their most successful hires tend to already be highly developed in their respective disciplines, allowing them to assess how to best leverage their abilities within the context of the company. The overwhelming majority of college students/graduates and run-of-the-mill gaming industry veterans wouldn't merit the slightest consideration, leaving Valve to chase after an inherently limited pool of exceptional people.
However, mentoring a group of young, highly self-motivated interns should be quite interesting, especially if there's robust experimentation with different teaching approaches. The ones that excel in Valve's unusual working culture will have their future educational trajectories influenced, and they could readily become vetted hires down the road.