Don't forget that in addition to code and art, you have data. Even if someone clones your codebase and invests time in recreating visual assets, they won't be able to draw on the player experiences you've been collecting. With data, you're able to identify trends and test new approaches smartly in order to maintain the edge; your would-be competitors wouldn't have much more to go on than their own intuition.

I won't claim to know what he thinks exactly -- maybe he's the alum you refer to? -- but I wonder if Samuel Clay (Newsblur, YC S12) would agree with that sentiment, given that he has published https://github.com/samuelclay/NewsBlur for all to see. I haven't committed to my own startup idea yet, but my plan for it does involve open-sourcing the code while keeping a custom corpus and its associated NLP models private.