I don't see what's stupid about that. Are you suggesting that anytime we want to talk about polyfills, we should instead write the following?

  #include "config.h"

  // ...

  #if !HAVE_FOOBAR
  void foobar(...)  // missing: implement ourselves
  {
  }
  #endif

Because that's nuts. This usage is actually more useful than the one that's a contraction of "polygon filling."