In the first few lines of the library .h file you linked, it tests a symbol it has not defined, ESP32 (#if defined(ESP32)) and alters the compilation according to the definedness of that symbol.
Since the compiler input will be one long file with all the #include files already "spliced" inline, you (or a previously included .h file) have the option to pre-define that variable to alter the compilation as this library writer provided.