This is the most clean, grokkable, readable C++ code I think I've ever read.

I feel like most open source C++ projects I read in non-game spaces are just a complete mess of indecipherable syntax.

Is this game code uniquely well written or am I often looking at the wrong codebases?

SerenityOS's codebase is extremely clean:

https://github.com/SerenityOS/serenity