> Hence it's an empirical fact that C and C++ have a superior approach to software scaling
I'm not convinced the "superior" follows. What I'm missing is a causal link between your first and second points - super large software could have been written in C or C++ for reasons other than the header/implementation split (especially if the super large software wasn't so super large at first and grew into its size instead of being planned for that size from the start), and somewhat conversely programming languages with header/implementation splits don't necessarily have corresponding super large software (e.g., OCaml, at least publicly?).
I'm not convinced the "superior" follows. What I'm missing is a causal link between your first and second points - super large software could have been written in C or C++ for reasons other than the header/implementation split (especially if the super large software wasn't so super large at first and grew into its size instead of being planned for that size from the start), and somewhat conversely programming languages with header/implementation splits don't necessarily have corresponding super large software (e.g., OCaml, at least publicly?).