although I still can't see any actual XSUB code

No, what you have there is sufficient.

...

int
return_one()

int
return_zero()
[download]

What you posted later is overkill.

...

int
return_one()
  CODE:
    RETVAL = return_one();
  OUTPUT:
    RETVAL

int
return_zero()
  CODE:
    RETVAL = return_one();
  OUTPUT:
    RETVAL
[download]