Hi! I don't know which guide you're looking at or which model iBook, so I don't know which step 28...could you possibly provide a link to the exact picture, and then describe the location of the rivet?
The topcase of an iBook should only have phillips head screws...does this rivet have a phillips head? Could it have been a phillips head, but maybe now it's stripped? I'm wondering if maybe someone put a non-standard screw into this machine while doing a previous repair?
Some generic suggestions -- you can use a dremel tool to cut the head off a screw, or cut a groove into it so that you can use a flathead screwdriver to get it out, etc.