This is a very strange interpretation.
When a piece of software that is released under a free license has a crucial non-free dependency, it is effectively non-free. As the software only works with the non-free dependency, but that dependency cannot be conveyed along with the free part, the whole thing cannot legally be distributed and thus is non-free.
Consider: there are also hardware platforms that have effectively ceased to exist over the years. If you wrote a program for one of these platforms, and it was open-source, would you now consider the program closed-source, because—given that the platform hardware doesn't exist any more—you can't run the software any more?
No. Why not? Because open source is about the source, not about the binary. 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.
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.