One "advantage" (if one wants to call that) is that the code would also compile as C++, while C99 has diverged enough from the common C/C++ subset that one cannot use all C99 features in C++ mode.
Slightly unrelated, but just thought I should mention: the sokol libraries are awesome!
C isn't a subset of C++ (and never was). But there's still a common subset of both the C and C++ languages which compiles both in a C compiler and a C++ compiler (and behaves the same at runtime despite slightly different C vs C++ semantics), and that common subset is what I call C/C++: the pidgin dialect that's neither quite C nor quite C++ but compiles as both.