I have never seen any multiple-data-point evidence presented to support it. Sure, various prominent entrepreneurs used computers as hobbyists (Gates, Zuckerberg), but does that apply to the typical programmer?
We all encountered students in classes who didn't study yet completely grokked their mathematics and algorithms coursework. Why is it so hard to believe that those quick learners use their time effectively at work?
Additionally, a multiplier to good engineering is strong communication and organizational skills; those can be enhanced through social recreational activity, and diminished by spending leisure time in solitude.
I'm not claiming either method is better, and intuitively the passionate engineer should win, but we shouldn't take it for granted until we get some actual information.
Edit: there seems to be some belief that I don't value experience. It is extremely important. We just shouldn't take it for granted that programming at home is as valuable as work experience.