These values are not in the range of meaningful values for a scaling operation.

I see that other platforms accept that in the context of the example program,

but I see that as a happy border effect an FLTK app should not rely on.

Why does the example program call fl_scale(0, 0) in the first place?

About how the Cairo backend of FLTK draw points :

it draws a filled rectangle of size 1 unit as shown by this code fragment

  void Fl_Cairo_Graphics_Driver::point(int x, int y) {
    rectf(x, y, 1, 1);
  }