The filtering phenomenon is obsolete; as you note, GM probably still cares because it's old fashioned, but modern companies don't care at all. Even if the filter exists/existed, that's hardly a sufficient or effective motivation to start a startup -- good startup founders don't seem like the type who'd just take a salaried gig and become complacent if it was offered to them.

There's probably some traits useful to startup founders that are more common amongst dropouts than the general population -- like a willingness to take responsibility for one's future into one's own hands.