It's true that if you just rely on free time to do work on a mod you'll never get it done.
Disagree. I'm lead coder for a fairly sizable and popular mod (4th in moddb awards 2007, honourable mention and 2nd unreleased 2006, 14th and 2nd unreleased 2005) and I get everything I need to done just in my spare time, juggling a full-time job, a full-time girlfriend and a full-time cat. What I think he really means is only antisocial types need apply.
Work expands to fill the time available. -- Cyril Northcote Parkinson, "The Economist", 1955