Well, that would be true if it was the same team doing the re-write and/or the original code was set in stone. Instead it was one guy. Stockfish is still heavil

Well, that would be true if it was the same team doing the re-write and/or the original code was set in stone. Instead it was one guy. Stockfish is still heavily worked on and the developers have access to CFish code. The reason they are not porting performance improvements is exactly that it wouldn't be idiomatic C++ (even though the whole project uses only minimal set of C++ features). One of his explicitly stated goals was to explore possible optimization issues of C versus C++ compilers. That is to see how much typical even light C++ abstractions cost in comparison to more close to the metal style.