I inspected the code.lua and find out that doing Game is Win isn't the same as Game is Defeat. I believe Game is Win triggers the 'hidden ending' in some ways because it calls a function called MF_wingame(), Defeat and Sink calls MF_endgame().
So the game actually closes either way, difference is that after Game is Win when you launch the game there is a "Thanks you for playing" at the top right corner, I think.