I'd modify that to say, that old programs often ask less of the system - although they tend to have assumptions about the environment (like the target processor) embedded/implied in the code. I think the single enhancement to C was adding strong typing and explicit typing in the function calls. By adding those two things to old code, I have brought them forward.
Similarly, I have taken modern code and my careful use of the preprocessor be able to get it run on as far back as the 5th edition without real hacks. And with what Warren and Phil did we even got pseudo C to compile back on V0.