The function drawRotatedBitmap doesn't return anything. So there is a mistake in "Inside Playdate with C" where it is written
LCDBitmap* playdate->graphics->drawRotatedBitmap(LCDBitmap* bitmap, int x, int y, float degrees, float centerx, float centery, float xscale, float yscale);
instead of
void playdate->graphics->drawRotatedBitmap(LCDBitmap* bitmap, int x, int y, float degrees, float centerx, float centery, float xscale, float yscale);