That's not even so much what I meant. The "game" isn't the engine; and so the "game"
is open source. You're free to modify and redistribute everything that constitutes the game. To
play the game, you need the game engine, similar to how running a program requires a machine (real or virtual) with a compatible ISA. But there's nothing forcing you to use a closed-source implementation of the game engine. If an open-source implementation of the engine doesn't exist, you're always free to write one.
Nobody ever says that a program being written for closed-source hardware necessarily implies that the program can't be open source. Because, of course, you can just write your own open VM for that same hardware. Saying that e.g. Gameboy Advance home-brew is all closed source, because the reference implementation of the hardware is closed source, is nonsensical. People are free to write, modify, and run GBA home-brew. Doing the last one just requires an emulator for most people, which is an everyday tool for programmers (given that many languages target abstract machines like the JVM or Erlang's BEAM, where the resulting bytecode ISA is only reified as an emulator.)
Nobody ever says that a program being written for closed-source hardware necessarily implies that the program can't be open source. Because, of course, you can just write your own open VM for that same hardware. Saying that e.g. Gameboy Advance home-brew is all closed source, because the reference implementation of the hardware is closed source, is nonsensical. People are free to write, modify, and run GBA home-brew. Doing the last one just requires an emulator for most people, which is an everyday tool for programmers (given that many languages target abstract machines like the JVM or Erlang's BEAM, where the resulting bytecode ISA is only reified as an emulator.)