Separating the interface from the implementation details? Isolating the server code from the module that uses it? Because the implementation is in the header, including it defines a bunch of macros that could easily collide with macros in the larger project.

Not knocking the overall effort here, but yes, "header only" libs are definitely an anti-pattern in real world C. I assume the author considers it a curiosity, a novelty, or something that would only be used in very limited circumstances that make such a design an advantage.