think about it this way -
if a 15x9+30 was available, while the rim might protrude, a narrow tyre would pull the top of the tyre under the guard.
if a 18x9+30 is fitted, the rim as a function of location from the guard will be further out (remembering that the guard does not sit at 90 degrees from the road surface) so will struggle to fit under the guard.
i can't explain it without a pic... sorry