> Why not? Because open source is about the source, not about the binary.

If you have the platform and the source, but you can't run the program, then I wouldn't consider it "open source". At least not in the usual sense of FOSS. It's not just about the source; it's about the freedom to study and modify the source, produce improved versions of the original binary, and distribute that binary and the source that created it.

The division between game and platform is lower in the stack than the engine.

> The source is a product in its own right, and valuable in its own right, whether or not it can be compiled into a working program.

I agree with that statement, but it's a different argument than you were making before.