If you want to be pedantic, there is a constexpr in the snippets, wont be accepted by the C compiler. More broadly though it is a C++ tutorial. I insisted on it because the author of algorithmica actually explains he chose C++ and perhaps sometime later C/Rust or something might be better. Should respect his POV.
You have to be 1/3 down the page to see any of this in the code examples. By that point, you would already have seen a number of examples that are valid C code and it is reasonable to expect the rest to be. At least, that was my experience.