According to Wikipedia[0], fish was originally created by Axel Liljencrantz in 2005. The commit history from GitHub does go back that far (I imagine it was imported somehow), so I assume it's complete. The first commit from ridiculous_fish is in 2011.

So if there is such a policy at Apple, I would expect that it doesn't prohibit contributing to already-public projects.

[0] http://en.wikipedia.org/wiki/Friendly_interactive_shell