I don't think that dpritchett was talking about open source vs closed source work. He was talking more about NDA-covered work, which could be on open or closed source code.

For most of the "normal" closed source work, it's easy to say that you worked on X product or feature, and can talk about some of the challenges that you encountered with it.

Contrast that with defense contract work that requires classified clearance. If you're a developer on that, you will most likely never be able to talk about it to most recruiters or interviewers before you retire. There is no way to actually talk about what you've done with the general public. That can hurt a career, especially if it puts a black hole in your experience on a resume.