They are basically using it instead of macros inside the header file.

Do you have a relevant example?

Essentially all the most important functions to interact with rings during runtime is static inline. The prep_* methods for sqes are so simple they are not problematic to reimplement, but all the other interactions necessary - submit, advance, peek, wait etc are all static inline, nested several layers. Those are not trivial and details have changed over time.

Thankfully that particular library do nowadays also provide a _ffi version of the .so as it is such an horror otherwise. But there are other libraries out there that don't do that, where it is almost as big of a hurdle to bind with.